Ancient art caves reopened

The paleolithic cave paintings of Altamira were at first dismissed as forgeries.

• "Sistine Chapel of paleolithic art" reopens to small groups


• Effect of visitors on cave's humidity, CO2 to be measured


• Murals discovered by amateur archaeologist and his daughter in 1879

(CNN) -- It's been called the "Sistine Chapel of paleolithic art," and it's been closed to the public for more than a decade -- until now.

Yesterday, five members of the public who entered a draw were given a guided tour of the extraordinary prehistoric paintings within the Altamira cave in northern Spain.

The vivid images, up to 20,000 years old, depict an extinct species of bison, wild boar and other animals, along with sprayed hand prints and engravings.

The visit of a lucky few to the twisting, 300-meter-long cave -- sealed in pristine condition by a rockfall for 13,000 years before its discovery in 1879 -- serves a wider purpose beyond their own amazement, however.

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Small groups drawn from visitors to the nearby museum on one randomly selected day each week until August will be used to test any destructive impact a human presence might have on the ancient paintings through changes in humidity, temperature, CO2 levels and microbiological contamination, the Guardian reports.

Wearing protective clothing, masks and shoes, the visitors get a tour of the cave lasting just over half an hour, only a portion of which is to be spent gazing at the rock art.

The results will help to determine whether the cave, in the Cantabria region, should be reopened to the public permanently -- a boon to the local economy but an outcome opposed by some scientists.

A sculpture in Cantabria represents one of the cave bison.

Dismissed as forgeries

Revealed in the late 19th century by an amateur archaeologist led to the site by his eight-year-old daughter, the charcoal and ochre animal studies and other figures at Altamira were at first dismissed as forgeries.

The images, in some cases exploiting the cave's natural contours to create an even more lifelike effect, were deemed too sophisticated to have been produced by ancient humans.

It was only 20 years later, when the Altamira paintings could be compared with other recent prehistoric finds, that their extreme antiquity was accepted -- and the understanding of early humans transformed.

READ: 11 things to know before visiting Spain

The Altamira cave was first closed to the public in 1977, when the body heat and CO2 from 3,000 daily visitors were found to be damaging the paintings.

The site was subsequently reopened to restricted numbers, creating a waiting list of up to three years.

In 2002, Altamira -- a Unesco World Heritage Site since 1985 -- was closed a second time when body heat, light and moisture were found to be causing a green mold to grow on the ancient works.

Campaign to reopen cave

Since then the Cantabrian regional administration has pushed for Altamira to be opened to the public once more.

The goal was to achieve a "balance" between preserving the rock art and making a vital part of Spain's heritage accessible, the director of the Altamira Museum, José Antonio Lasheras, told CNN.

However, the Spanish government's main research body opposes the reopening of the cave.

"The consequences of doing so are incalculable," Sergio Sánchez Moral, lead researcher at the Spanish National Research Council, told Publico.es.

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For the past decade, visitors to Altamira have had to be content with replicas of the paintings -- mainly those in the cave's main chamber -- located a short walk away in in the museum.

"The original [paintings] can be thrilling because the feeling of entering the cave is difficult to reproduce," Lasheras said.

"But people have also felt excited going through the replica."

The museum also has a permanent exhibition called "The Times of Altamira," including furniture made of animal bone and antler, depicting life during the paleolithic era.

Museo de Altamira , Avda. Marcelino Sanz de Sautuola, 39330 Santillana del Mar, Cantabria, Spain; +34 942 818 005/815

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11 vanishing wildlife experiences

Poaching, war and deforestation threaten magnificent sights such as the annual pilgrimage of wildebeest across the African savannah.

Climate change is melting polar bears' Arctic habitat -- you can still see them in the wild in places such as Canada's Hudson Bay.

The world's most endangered big cats have disappeared from 90% of their historic territory. India and Nepal present rare chances to see tigers in the wild.

Australia's Great Barrier Reef is shrinking. Snorkeling over it may become increasingly rare.

Poaching and disease are decimating the world's largest primate -- you can still watch them in their habitat in the Republic of the Congo.

You can spot the animals that inspired Darwin's "The Origin of Species" on a Galápagos cruise.

One of man's closest animal relatives could be extinct in a decade. In Borneo you can visit them in rehabilitation centers or the wild.

Intense egg collection, fisheries bycatch and light pollution have forced species such as the leatherback, hawksbill and green turtles onto endangered lists.

Botswana offers one of the best chances to see rhinoceros, the most endangered of the legendary Big Five African game species.

Demand for their meat and fins presents an enormous threat to whale sharks. You can still swim with them off the Philippines.

There are only around 1,600 giant pandas left in the wild.

• You can swim alongside rare whale sharks in the Philippines


• African wildebeest among great migrating animals under threat


• Gorillas can still be seen in Congo -- for now

(CNN) -- Earth is in the grip of the "sixth great extinction crisis," say some scientists, with up to 2,000 species dying out each year.

These 11 great wildlife experiences could disappear within your lifetime.

1. Witnessing a great animal migration

Ancient rock art images of animals moving across the African savannah indicate that humans have been marveling at animal migrations for more than 20,000 years.

But poaching, agriculture, war and deforestation are encroaching on wildlife corridors around the world, placing these animal movements -- from Africa's wildebeest migration to the annual pilgrimage of North American bison -- under threat.

How to do it: GAdventures pairs good wildlife access with creature comforts on its Kenya Safari Experience tour (eight days from $3,490 per person; +1 888 800 4100), taking in Kenya's Lake Nakuru, Amboseli National Park and the Masai Mara, where Africa's wildebeest end their migration.

Polar bear numbers could shrink by two-thirds by mid-century.

2. Coming eye-to-eye with a polar bear

While hunting is still allowed by some Arctic-dwelling native communities, it's climate change that poses the largest threat to polar bears.

Latest data from the International Union for the Conservation of Nature (IUCN) shows that eight of the 19 sub-populations of polar bear that roam the Arctic are in decline.

Scientists project that two-thirds of all polar bears could disappear by 2050.

Due to its southern exposure, Canada's Hudson Bay is one of the bear habitats most threatened by climate change -- increasing temperatures are forcing bears off the ice earlier each year.

How to do it: Churchill Wild (+1 866 846 945) is one of a handful of operators on the western shores of Hudson Bay that facilitate multi-day bear watching tours starting at around $3,000 per person.

3. Counting the stripes on a tiger

Tigers once roamed freely across Asia.

Today the world's most endangered big cats have disappeared from 90% of their historic range.

Just six of nine known tiger species are thought to remain, with an estimated 3,200 animals left in the wild.

READ: Best places in Africa to sleep beneath the stars

While it's extremely rare to spot the elusive beasts in their natural habitat (the South China tiger, for example, hasn't been seen since the early 1970s), protected reserves in India and Nepal provide the best opportunities.

How to do it: Intrepid Travel offers a Chitwan Extension (four days from $430 per person; +1 800 970 7299) tour to Nepal's Chitwan National Park from Kathmandu.

Once a royal hunting reserve, Chitwan is home to elephant, rhino, crocodile, monkeys, deer and about 100 Bengal tigers.

Tracts of the world's largest reef system are in danger.

4. Swimming amid live coral on Australia's Great Barrier Reef

The Great Barrier Reef, the world's largest reef system, is in peril.

Temperature fluctuations are primarily to blame for the disappearance of half its coral since 1985 and the Australian Institute of Marine Science predicts that less than a quarter could be left within a decade.

Following plans by the Australian government to industrialize parts of the reef, the World Heritage Committee passed a recommendation to consider listing the UNESCO site as "in danger" at a meeting in June 2014.

How to do it: Calypso Snorkel and Dive (+61 7 4099 6999) offers daily snorkeling and diving day trips to the reef from the Far North Queensland resort town of Port Douglas starting from $174 per person.

5. Tracking gorillas in the mist

Endemic to the forests of central Africa, both species of the world's largest primate have been decreasing in numbers for decades largely due to poaching and disease.

Worryingly, a 2010 United Nations report suggested that these gentle giants of the forest might disappear from large parts of the Congo Basin, one of their primary habitats, by the mid-2020s.

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How to do it: Wilderness Collection offers gorilla tracking opportunities during a six-day tour, from $6,474 per person, split between two luxury Odzala camps (+27 11 807 1800) in an area of the Republic of the Congo that's home to a high density of western lowland gorilla.

Chimpanzee, leopard, golden cat and a large number of other primate and bird species are also present.

6. Seeing the creatures of Galápagos as Darwin saw them

Remaining virtually undiscovered for centuries did much to preserve the living showcase of evolution described by Charles Darwin in "The Origin of Species."

But Ecuador's best known ecosystem remains fragile.

The last remaining Pinta Island tortoise, fondly known as Lonesome George, died in 2012, adding to the list of several other species that have died out since Darwin's 1835 visit.

How to do it: While it's possible to fly to Galapagos from mainland Ecuador and visit several islands independently, you can only reach the bulk of protected habitats on heavily regulated organized cruises.

You can expect to pay from around $1,500 for a last-minute, week-long cruise organized in the capital Quito or Puerto Ayora on the island of Santa Cruz.

Prices double (or more) for the convenience of booking ahead through an international company such as Ecoventura (+1 800 633 7972).

Illegal pet trade is one of orangutans' biggest threats.

7. Hanging around with orangutans

Sharing 98% of our DNA, orangutans are one of the closest relatives to man, though if predictions are accurate, Asia's only two great ape species could be extinct in the wild within the next decade.

Conservation groups such as Borneo Orangutan Survival have bolstered numbers through rehabilitation and release programs, but the illegal pet trade, trafficking and habitat destruction still present the greatest threats to the survival of orangutans on the only two islands they inhabit: Borneo and Sumatra.

READ: How to travel quietly

How to do it: Gecko's Adventures include a visit to two rehabilitation centers in Malaysian Borneo on their Orangutanland tour (18 days from $2,500; +44 0808 274 5981).

The trip includes a cruise down Kinabatangan River, where you might spot orangutans, proboscis monkeys, macaques and elephants in the wild.

8. Watching sea turtles nest

Scientists have been debating for decades why -- not to mention how -- sea turtles return to the same beach every year to lay their eggs.

But the phenomenon may soon be a thing of legend thanks to the ravages of intense egg collection, fisheries bycatch and light pollution, which have forced species such as the leatherback, hawksbill and green turtles onto endangered lists.

How to do it: You can sign up for a volunteering stint with an organization such as Earthwatch (nine-day Costa Rica program from $2,800 per person; +1 978 461 0081) during which you'll help safeguard turtle eggs and assist new hatchlings.

Minus rhinos, Big Five will become Big Four.

9. Spotting Africa's Big Five

European game hunters and the illegal wildlife trade may soon reduce Africa's most prestigious group of game animals from the legendary Big Five to four.

While lion, African elephant, Cape buffalo and leopard populations are relatively stable, rhinos are in serious trouble.

Poaching has increased dramatically during the past few years, with 800 rhinos killed illegally in South Africa in 2013 alone.

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Last year, the International Union for Conservation of Nature (IUCN) declared the western black rhino officially extinct.

How to do it: All members of the Big Five can still be found in Botswana, Uganda, Namibia, Ethiopia, South Africa, Kenya, Tanzania, Zimbabwe, Malawi and the Democratic Republic of the Congo.

If you're particularly eager to see rhinos, Intrepid Travel's Okavango Experience (nine days from $1,155 per person; +1 800 970 7299) includes a dusk game drive in Botswana's Khama Rhino Sanctuary for a chance to spot both white and black rhino.

10. Swimming alongside whale sharks

Nicknamed "dominos" for their distinctive markings, the world's largest fish are among the most placid of the shark species.

Unfortunately, demand for whale shark meat, fins and oil, on top of unregulated tourism, remains an enormous threat to this vulnerable species.

How to do it: A WWF-sponsored ecotourism project in the Philippine village of Donsol allows tourists to swim with whale sharks while following strict rules to avoid disturbing them.

Donsol EcoTour (+63 2 576 5934) offers snorkeling packages with gear and a trained guide from around $25 per person.

11. Be awed by a giant panda in the wild

They're China's natural treasure, but threats from humans have left just 1,600 giant pandas in the wild.

High in the mountains of western China, the rarest member of the bear family plays a critical role in facilitating the growth and vegetation of the bamboo forests these placid beasts inhabit.

How to do it: Given the difficulty of spotting giant pandas in the wild, only a small number of tour operators exist.

The WWF-partnered Natural Habitat Adventures' Wild Side of China tour (13 days from $9,595 per person; +1 303 449 3711) includes a visit to China's Chengdu Giant Panda Base and rare access to a panda sanctuary to search for them in the wild.

READ: Namibia's Skeleton Coast -- The most pristine place in Africa?

Sarah Reid is a travel writer who has contributed to Lonely Planet publications, Cosmopolitan and Spa Secrets.

Interactive guide: Moroccan food

Medvedev: New revolution possible

A woman waits in front of unidentified men in military fatigues blocking a base of the Ukrainian frontier guard unit in Balaklava, Ukraine, on March 1. Ukraine suspects Russia of sending new troops into Crimea and provoking separatist tensions in the region. Crimea is an autonomous republic of Ukraine with an ethnic Russian majority. It's the last large bastion of opposition to Ukraine's new political leadership after President Viktor Yanukovych's ouster.

U.S. President Barack Obama talks on the phone in the Oval Office with Russian President Vladimir Putin about the situation in Ukraine, March 1

Troops stand guard in Balaklava, Crimea, on Saturday, March 1.

Heavily armed soldiers displaying no identifying insignia maintain watch in Simferopol, Ukraine, on March 1.

People gather around a coffin of a man who was killed during clashes with the riot police in Kiev's Independence Square.

Armed men in military uniform block a Ukrainian military base in Balaklava on March 1.

Pro-Russian activists hold Russian flags during a rally in the center of Donetsk, Ukraine, on March 1.

Pro-Russian activists clash with Maidan supporters as they storm the regional government building in Kharkiv on March 1.

A protester stands at a memorial on March 1 for the people killed in clashes with the police at Kiev's Independence Square.

Armed men patrol outside the Simferopol International Airport in Ukraine's Crimea region on Friday, February 28. Simferopol is the regional capital of Ukraine's Crimea.

An image provided to CNN by a local resident shows Russian tanks on the move in Sevastopol, Ukraine.

Russian troops block a road February 28 toward the military airport in Sevastopol, Ukraine, on the Black Sea coast. The Russian Black Sea Fleet is based at the port city of Sevastopol. Ukraine suspects Russia of fomenting tension in the autonomous region of Crimea, which might escalate into a bid for separation by its Russian majority.

Armed men stand guard in front of a building near the Simferopol airport on February 28.

An armed man wearing no identifying military insignia patrols outside Simferopol International Airport on February 28.

Police stand guard outside the Crimea regional parliament building in Simferopol on Thursday, February 27. Armed men seized the regional government administration building and parliament in Crimea.

Police intervene as Russian supporters gather in front of the parliament building in Simferopol on February 27.

A man adds fuel to a fire at a barricade on Kiev's Independence Square on February 27. Dozens of people were killed last week during clashes between security forces and protesters.

Pro-Russia demonstrators wave Russian and Crimean flags in front of a local government building in Simferopol on February 27.

Barricades in front of a government building in Simferopol on February 27 hold a banner that reads: "Crimea Russia." There's a broad divide between those who support the pro-Western developments in Kiev and those who back Russia's continued influence in Crimea and across Ukraine.

Protesters stand in front of a government building in Simferopol on February 27. Tensions have simmered in the Crimea region since the Ukraine president's ouster.

Protesters in support of the president's ouster rally in Kiev's Independence Square, which has been the center of opposition, on Wednesday, February 26.

Security forces stand guard during clashes between opposing sides in front of Crimea's parliament building in Simferopol on February 26.

Pro-Russian demonstrators, right, clash with anti-Russian protesters in front of a government building in Simferopol on February 26.

A police officer gets pulled into a crowd of Crimean Tatars in Simferopol on February 26. The Tatars, an ethnic minority group deported during the Stalin era, is rallying in support of Ukraine's interim government.

A man places flowers at a barricade near Kiev's Independence Square on February 26.

A woman holds a photograph of a protester killed during the height of tensions on February 26 in Kiev.

Police guard a government building in Donetsk in eastern Ukraine on February 26.

Protesters remove a fence that surrounds Ukraine's parliament in Kiev on February 26.

People sing the Ukrainian national anthem at Kiev's Independence Square on Monday, February 24.

Gas masks used by protesters sit next to a barricade in Independence Square on February 24.

A woman cries February 24 near a memorial for the people killed in Kiev.

People wave a large Ukrainian flag in Independence Square on Sunday, February 23.

Two pro-government supporters are made to pray February 23 in front of a shrine to dead anti-government protesters.

A man and his daughter lay flowers at a memorial for protesters killed in Independence Square.

Former Prime Minister Yulia Tymoshenko speaks at Independence Square on Saturday, February 22, hours after being released from prison. Tymoshenko, considered a hero of a 2004 revolution against Yanukovych, was released after 2½ years behind bars.

Tymoshenko is greeted by supporters shortly after being freed from prison in Kharkiv on February 22.

A protester guards the entrance to Yanukovych's abandoned residence outside Kiev on February 22.

Anti-government protesters guard the streets next to the presidential offices in Kiev on February 22.

Anti-government protesters drive a military vehicle in Independence Square on February 22. Many protesters said they wouldn't leave the square until Yanukovych resigned.

Ukrainian lawmakers argue during a session of Parliament on Friday, February 21.

Men in Kiev carry a casket containing the body of a protester killed in clashes with police.

Protesters cheer after news of an agreement between the opposing sides in Kiev on February 21.

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• NEW: "What Russia is doing now in Ukraine violates the principles of the U.N. charter" - NATO chief


• Ukrainian PM says Russian actions are "a declaration of war"


• Putin says Russia reserves the right to defend its interests and people


• Obama made clear violating Ukraine's sovereignty would impact Russia's standing

Have you witnessed the crisis in Ukraine, particularly in Crimea? Share your experiences, but please stay safe.

Kiev, Ukraine (CNN) -- Ukraine's new Prime Minister Arseniy Yatsenyuk warned Sunday his crisis-hit country was on the "brink of disaster," accusing neighbor Russia of declaring war.

Ukraine's shaky new government mobilized troops and called up military reservists Sunday, even as the defense minister said Kiev stood no chance against Russian troops in a rapidly escalating crisis that has raised fears of a conflict.

Amid signs of Russian military intervention in Ukraine's Crimean peninsula, Russian generals led their troops to three bases in the region Sunday demanding Ukrainian forces surrender and hand over their weapons, Vladislav Seleznyov, spokesman for the Crimean Media Center of the Ukrainian Defense Ministry, told CNN.

Speaking by phone, he said Russian troops had blocked access to the bases, but added "there is no open confrontation between Russian and Ukrainian military forces in Crimea" and that Ukrainian troops continue to protect and serve Ukraine.

Russia OKs military force in Ukraine

Kiev: Russia's move is direct aggression

"This is a red alert. This is actually a declaration of war in our country," Yatsenyuk said.

Speaking in a televised address from the parliament building in Kiev, Yatsenyuk called on Russian President Vladimir Putin to "pull back the military and stick to international obligations."

"We are on the brink of disaster," he said.

In Brussels, Belgium, NATO ambassadors were scheduled to hold an emergency meeting on Ukraine.

"What Russia is doing now in Ukraine violates the principles of the U.N. charter," NATO Secretary General Anders Fogh Rasmussen told reporters.

"Russia must stop its military activities and threats," Rasmussen said, adding, "we support Ukraine's territorial integrity and sovereignty. ... We support the rights of the people of Ukraine to determine their own future without outside interference."

Escalating crisis

Ukraine's parliament met behind closed doors Sunday. At the closing of the session, acting Defense Minister Ihor Tenyuh said Ukraine does not have the military force to resist Russia, according to two parliamentary members present at the meeting. Tenyuh called for talks to resolve the crisis with Russia, they said.

The Ukrainian National Security Council has ordered the mobilization of troops, as Putin appeared to dismiss warnings from world leaders to avoid military intervention in Crimea, a senior Ukrainian official, Andriy Parubiy, said.

Masked gunmen occupy Crimea

Ukranian airspace shut down amid crisis

He also said the Defense Ministry was calling for reservists to register at the local and regional headquarters to be on standby if needed.

A sense of escalating crisis in Crimea -- an autonomous region of eastern Ukraine with strong loyalty to neighboring Russia -- swirled Saturday night, with U.S. Secretary of State John Kerry condemning what he called "the Russian Federation's invasion and occupation of Ukrainian territory."

Russia has not confirmed it deployed thousands of troops to the region following reports that armed, Russian-speaking forces wearing military uniforms -- without insignia -- patrolled key infrastructure sites.

It was the latest in a series of fast-moving developments that saw Russia's parliament sign off on Putin's request to send military forces into Ukraine, raising the stakes in the escalating brinksmanship. Putin cited in his request a threat posed to the lives of Russian citizens and military personnel based in southern Crimea. Ukrainian officials have vehemently denied Putin's claim.

According to a tweet from the official Russian government account Sunday, Russian Prime Minister Dmitry Medvedev discussed the crisis in Ukraine in a telephone call with Yatsenyuk.

According to a second tweet, Medvedev said Russia is interested in maintaining stable and friendly relations with Ukraine but reserves the right to protect the legitimate interests of its citizens and military personnel stationed in Crimea.

Path to war?

In Kiev, thousands of people rallied in the central Independence Square, cradle of Ukraine's three-month anti-government protests that led to President Viktor Yanukovych's ouster last week.

A crowd held up signs reading "Crimea, we are with you" and "Putin, hands off Ukraine."

Putin's move prompted world diplomats to call for a de-escalation of tensions that have put the two neighbors on a possible path to war and roiled relations between Russia and the United States.

In what appeared to be an illustration of the growing schism between the two world powers, U.S. President Barack Obama and Putin spoke for 90 minutes, with each expressing his concern over the mounting crisis, according to separate statements released by their governments.

According to the Kremlin, Putin told Obama that Russia reserves the right to defend its interests in the Crimea region and the Russian-speaking people who live there.

The Russian government said in a statement that, in reply to U.S. concerns over the possibility of the use of Russian armed forces in Ukraine, Putin "drew his attention to the provocative and criminal actions on the part of ultranationalists who are in fact being supported by the current authorities in Kiev."

According to a statement released Saturday by the White House, Obama "made clear that Russia's continued violation of Ukraine's sovereignty and territorial integrity would negatively impact Russia's standing in the international community."

Lean to the West, or to Russia?

Ukraine, a nation of 45 million people sandwiched between Europe and Russia's southwestern border, has been plunged into chaos since the ouster a week ago of Yanukovych following bloody street protests that left dozens dead and hundreds wounded.

Ukraine has faced a deepening split, with those in the west generally supporting the interim government and its European Union tilt, while many in the east prefer a Ukraine where Russia casts a long shadow.

Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.

Ukraine's acting President Oleksandr Turchynov took to the airwaves late Saturday to warn that any Russian military intervention would lead to war.

'The troops are already there'

The crisis set off alarm bells with the world's diplomats, with Ukrainian Ambassador to the U.N. Yuriy Sergeyev calling on member nations of the Security Council to take a stand against what he called Russia's "clear act of aggression.''

"The troops are already there, and their number is increasing every hour," Sergeyev said during an emergency meeting of the Security Council.

Russia now has 15,000 troops Crimea, Yegor Pyvovarov, the spokesman for the Ukrainian mission at the United Nations, told CNN ahead of Saturday's session of the Security Council. He did not say how Ukraine arrived at that number, or whether that included troops already stationed at a Russian base in the region.

Vitaly Churkin, Russia's ambassador to the United Nations, rejected Ukraine's calls to stop Russian intervention. "We can't agree with this at all," he said.

He blamed members of the European Union for causing the bloody street demonstrations in Ukraine.

"It's a difficult situation in the past few hours," Churkin said, claiming that there were Ukrainian forces from Kiev en route to to overthrow the local pro-Russian governments in eastern Ukraine and Crimea and establish new ones that would enforce the power of the new Ukrainian government.

Churkin has said reports of Russian troops taking charge of positions on the ground were rumors and noted that rumors "are always not true."

Crimea's pro-Russian leader asked for help

The Russian parliament vote Saturday came on the day that the newly installed pro-Russian leader of Crimea, Sergey Aksyonov, asked Putin for help in maintaining peace on the Black Sea peninsula, where Russia's fleet is based at Sevastopol.

Security forces "are unable to efficiently control the situation in the republic," he said in comments broadcast on Russian state channel Russia 24. Aksyonov was installed as the region's premier after armed men took over the Crimean parliament building on Thursday.

Aksyonov said that a referendum on greater Crimean autonomy, originally set for May 25, would be moved to March 30.

Yatsenyuk called the Russian presence in Crimea a provocation.

"Ukraine will not be provoked, we will not use force. We demand that the government of the Russian Federation immediately withdraw its troops and return to their home bases," he said during a televised Cabinet meeting.

Airspace in the region reopened Saturday, a day after Ukraine accused Russian Black Sea forces of trying to seize two airports in Crimea but said Ukrainian security forces had prevented them from taking control.

Groups of armed men, dressed in uniforms without identifying insignia, patrolled the airports in Simferopol and the nearby port city of Sevastopol. The men remained at the airports Saturday, but Yevgey Plaksin, director of the airport in Simferopol, said airport services were working.

Obama: Warning to Russia

Senior White House officials say they are looking at a wide range of possible economic and diplomatic measures to present to Obama that would show Putin there is a cost to his actions in Ukraine.

The White House has already announced the United States will suspend participation in preparatory meetings for the G-8 summit that will bring world leaders together in June in Sochi, Russia.

"Going forward, Russia's continued violation of international law will lead to greater political and economic isolation," according to a statement released by the administration.

Pressure has been mounting on Russia as leaders from the EU and the UK joined an international outcry, with EU High Representative Catherine Ashton deploring Russia's "unwarranted escalation of tensions." British Foreign Secretary William Hague was scheduled to travel to Kiev on Sunday.

During a telephone call with Putin, U.N. Secretary General Ban Ki-Moon said he told the Russian leader that it was crucial to "restore calm and proceed to an immediate de-escalation of the situation."

"Cool heads must prevail and dialogue must be the only tool in ending this crisis," he said.

CNN's Victoria Eastwood and Diana Magnay reported from Simferopol, Ukraine; Ian Lee, Ingrid Formanek and Victoria Butenko from Kiev, while Marie-Louise Gumuchian and Chelsea J. Carter wrote from Atlanta. CNN's Richard Roth, Laura Smith-Spark, Tom Watkins, Sara Mazloumsaki, Alla Eshchenko, Arkady Irshenko, Radina Gigova and journalist Azad Safarov contributed to this report.