- NEW: U.S. official: Russian aircraft enter Ukrainian airspace on several occasions
- Ukraine official announces "second stage" of operation against militants in Slavyansk
- President Barack Obama says targeted sanctions against Russia are "ready to go"
Kiev, Ukraine (CNN) -- U.S. President Barack Obama threatened Russia on Friday with new sanctions over its actions in Ukraine as the Ukrainian Prime Minister also came out with tough words against Moscow.
The United States could announce further sanctions as early as next week, U.S. officials say.
Russian attempts to start conflict in Ukraine will lead to military conflict in Europe, Prime Minister Arseniy Yatsenyuk warned.
He also charged Russia with aiming to occupy his country "militarily and politically," and said Moscow wants to start a third world war.
His strong statement came amid a sharp escalation in tensions.
Russia's defense chief ordered new military drills Thursday near the border with eastern Ukraine after Ukrainian forces said they killed five pro-Russian militants in an operation to clear roadblocks near the city of Slavyansk.
Serhiy Pashinskiy, chief of staff to acting Ukrainian President Oleksandr Turchynov, said Friday that the drills had now entered its "second stage," aimed at encircling Slavyansk and cutting off additional supplies or support.
And on Friday afternoon, Pentagon spokesman Col. Steve Warren said Russian aircraft entered Ukrainian airspace on several occasions over the previous 24 hours. He urged Russia "to take immediate steps to de-escalate the situation."
Yatsenyuk said he'd asked Russia to explain the presence of its troops on Ukraine's border within 48 hours -- but 24 hours later, Kiev is still waiting for an official response.
At the same time, he said, the world is lined up with Kiev in its efforts to restore stability and preserve "the system of world security that Russia wants to destroy."
Just as Kiev announced the second phase of its so called "anti-terrorism operation," behind the barricades of the security services building, at the command and control center, the pro-Russian forces are confident they can rebel any sort of attack.
"We have 2,000 fighters at the ready and plenty of weapons," says Evgeniy Gorbik, a former military man turned spokesman for pro-Russian militants.
On Friday, Ukraine's Interior Ministry reported that 13 people on board a bus -- including foreign observers and military officials -- were detained at the entrance of Slavyansk. The ministry said the captors were unknown, but that the detainees were taken to a security office that is under the control of pro-Russian separatists.
The detainees are seven European representatives from the Organization for Security and Co-operation, five Ukrainian military representatives and a bus driver, the ministry said.
"We strongly condemn the fact that the members of the Organization for Security and Co-operation mission were taken hostage by pro-Russian separatist forces, this is completely unacceptable," Ukrainian Deputy Foreign Minister Danylo Lubkivsky told CNN.
Negotiations were ongoing to secure their release, the ministry reported.
"We also demand and call upon Russian authorities to exercise all their influence to release the hostages. We count on the solidarity between American and European parties in deescalating the situation and stop the aggressor. That's our principle stance, we would like to avoid any casualties, any victims and any conflicts," added Lubkivsky.
U.S. pressure
Meanwhile, top U.S. officials have vowed more punitive international actions targeting Russia's economy if Moscow, in their view, continues to escalate the situation.
The war of words between the United States and Russia is growing increasingly intense.
In a phone call Friday, Obama and European leaders agreed that Russia has failed to do its part to de-escalate tensions, the White House said.
The Russians have not supported a deal reached last week to decrease tensions, nor have they called on militant groups to put down their weapons, the White House said. To the contrary, Russia has made things worse through its rhetoric and military exercises, it said.
At a news conference Friday in South Korea, Obama said there will be targeted sanctions that are "ready to go."
"I think it's important for us not to anticipate that the targeted sanctions that we're applying now necessarily solve the problem," he said.
"What we've been trying to do is continually raise the costs for Russia of their actions while still leaving the possibility of them moving in a different direction. And we will continue to keep some arrows in our quiver in the event that we see a further deterioration of the situation over the next several days or weeks."
Obama also praised the unity of other countries in condemning Russian "meddling" in Ukraine.
West threatens Russia with more sanctions
According to senior U.S. officials, new sanctions could target key Putin allies, high-profile Russian oligarchs and institutions and companies.
The new sanctions "will be an escalation," one official said.
U.S. Secretary of State John Kerry called Russia's actions in Ukraine "a full-throated effort to actively sabotage the democratic process through gross external intimidation."
"If Russia continues in this direction," he added, "it will not just be a grave mistake, it will be an expensive mistake."
Russia also ramped up its rhetoric Thursday.
President Vladimir Putin told Russian state media that "if the Kiev regime has started to use the army against the population inside the country, it, beyond any doubt, is a very serious crime."
This, he said, would have consequences for Ukraine's interim leaders and for relations between the two countries.
Moscow argues that the government in Kiev, which took power after ousted pro-Moscow President Viktor Yanukovych fled Ukraine in February, is illegitimate.
On Friday, the International Criminal Court said chief prosecutor Fatou Bensouda had opened a preliminary investigation into alleged crimes committed in Ukraine while Yanukovych was in power.
The investigation will look at the period from November 21 to February 22, when street protests against Yanukovych's government took place that resulted in bloody clashes with security forces.
Volatile situation
Only a week has passed since foreign ministers for Ukraine, Russia, the United States and the European Union agreed to a deal in Geneva, Switzerland, aimed at easing the crisis.
But that pact, which called for all sides to refrain from violence and for illegal militia groups to disarm and leave occupied buildings, appears to have faltered, if not failed.
Pro-Russian militants remain holed up in government buildings in around a dozen cities in eastern Ukraine.
And efforts by the Ukrainian security services to dislodge them risk provoking more violence or perhaps direct Russian intervention.
Although Ukraine's Interior Ministry said five pro-Russian militants died in the operation Thursday near Slavyansk, separatists who spoke to CNN at one of the targeted roadblocks said only one person had died.
Slavyansk has been at the center of much of the tension.
Pashinskiy, the Ukrainian President's aide, said Friday that the militants there were equipped with new Russian weapons and that intelligence indicated they had set up bases in civilian sites, a kindergarten and hospital.
The evidence suggests, he said, "that terrorist actions in the east are directly managed by Russian special agents, that there are acting Russian troops in Slavyansk and that the so-called separatist groups are being coordinated by Russia directly."
Nearby in Kramatorsk, Russia's state-run RIA Novosti news agency reported gunfire at an airfield that Ukrainian forces retook from separatist protesters this month.
But the head of Ukraine's anti-terror operations unit, Vasil Krutow, told CNN that a helicopter at the airfield had exploded after a shot hit its fuel tank. He said the crew's commander was injured as a result.
An explosion was also reported overnight at a police checkpoint in the southwestern city of Odessa.
Competing claims
The United States has accused Russia of supporting, arming and coordinating the separatist militants -- an allegation Moscow denies, saying these are simply local "self-defense" units. NATO and the United States have also voiced concern over an estimated 40,000 Russian troops massed near the Ukrainian border.
Russia in turn accuses the United States of "running the show" in Ukraine and says it must do more to hold Kiev to its side of the Geneva deal, saying the interim government must disband right-wing ultranationalist groups.
Obama said Friday that the Ukrainian government had been acting in accordance with the Geneva agreement, including offering amnesty to those who will lay down their arms.
"What we have not seen is Russia speaking out clearly, condemning the pro-Russian militias that have taken over these buildings," he said.
Kerry sounded a similar note Thursday, praising what he called positive efforts by Ukraine's government to implement the Geneva deal while rapping Moscow for having "refused to take a single concrete step in the right direction."
Economic impact
Unless and until additional U.S. sanctions are announced, it's hard to judge how much they will hurt Russia. Previous rounds targeted individual lawmakers and businessmen seen as close to Putin as well as a bank, Rossiya, believed to serve the Russian President and senior officials. The European Union has also imposed its own asset freezes and visa bans.
Russia increased interest rates for a second month running Friday in an effort to limit the economic damage of rising tensions over Ukraine. The move came hours after ratings agency Standard & Poor's cut Russia's credit rating to one notch above junk, citing a flight of capital from the country.
The Russian ruble lost more ground Friday, taking its losses against the dollar to 8.4% since the start of the year. Russia's benchmark Micex index also slipped by 0.7%, extending its decline so far this year to 14.5%.
Russian Prime Minister Dmitry Medvedev acknowledged this week that the sanctions, coupled with other factors, were affecting Russia's economy.
"We do believe that by more economic pressure, we can stop the aggressor," said Danylo Lubkivsky, Ukrainian Deputy Foreign Minister.
Any further sanctions could hurt the European Union as well as Russia, since the two have deep trade ties and nearly a third of Europe's natural gas comes from Moscow.
Putin condemned the Ukrainian government's "attempts to use the army against civilians" in a phone call Friday with German Chancellor Angela Merkel, the Kremlin said, according to RIA Novosti.
The two leaders noted the importance of holding talks soon between Russia, the EU and Ukraine on the security of gas supplies as soon as possible, the news agency said.
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CNN's Gul Tuysuz and journalist Victoria Butenko reported from Kiev, and CNN's Laura Smith-Spark wrote and reported in London. CNN's Barbara Starr, Elise Labott, Boriana Milanova, Diana Magnay, Antonia Mortensen, Ben Brumfield and Mark Thompson contributed to this report.
