- Sen. Robert Menendez calls for the arming of Ukrainian forces.
- Menendez says Russia has invaded Ukraine and it's time to help the Ukrainians fight back.
- Ukraine's President Petro Poroshenko says conflict is inching toward "full-scale war."
- Russia is still denying it has sent troops and weapons over the border.
Mariupol, Ukraine (CNN) -- A U.S. lawmaker is calling for the arming of Ukrainian forces engaged in fighting Russian troops along the nation's eastern border with Russia.
Sen. Robert Menendez, D-New Jersey, is in the Ukrainian capital, Kiev, on a fact-finding mission into the monthslong conflict there between pro-Russian rebels and the Ukrainian military.
Appearing on CNN's "State of the Union" on Sunday, Menendez called this a "watershed moment" in the conflict. He said it's time to recognize what's really happening in eastern Ukraine -- not a rebel uprising, but a Russian invasion.
"And we must recognize it as that," said Menendez, chairman of the Senate Foreign Relations Committee. "When I read the headlines back at home that suggest rebels are advancing in different parts of eastern Ukraine, it's not rebels -- it's Russian soldiers."
"We should be providing the Ukrainians with the type of defensive weapons that will impose a cost upon (Russian President Vladimir) Putin for further aggression."
Putin: Don't mess with us, we're nuclear 
Why did Ukrainian forces leave border? 
Ukrainian troops evacuate from the rebel-held town of Starobesheve, eastern Ukraine, on Saturday, August 30. Ukrainian government forces have been battling pro-Russian rebels near Ukraine's eastern border with Russia. The fighting has left more than 2,000 people dead since mid-April, according to estimates from U.N. officials. 
A Ukrainian loyalist fighter from the Azov Battalion stands guard on a hill on the outskirts of Mariupol on August 30. 
A pro-Russian rebel listens to the news on a transistor radio in the town of Novoazovsk, in eastern Ukraine, on Friday, August 29. 
Ukrainian soldiers stop on a roadside as they wait for the start of their march into Mariupol, Ukraine, on Wednesday, August 27. 
A man opens a box on August 27 filled with rocket-propelled grenades left by the Ukrainian army in Starobecheve, Ukraine, after pro-Russian forces took control of the town. 
A pro-Russian rebel walks through a local market damaged by shelling in Donetsk, Ukraine, on Tuesday, August 26. 
Ukrainian servicemen of the volunteer battalion Azov leave for Novoazovsk, Ukraine, on August 26. 
Pro-Russian separatists escort captured Ukrainian army soldiers in a central square in Donetsk, Ukraine, on Sunday, August 24. 
People yell as Ukrainian prisoners are paraded through Donetsk in eastern Ukraine on August 24. 
A pro-Russian rebel delivers a speech atop a damaged Ukrainian armored personnel carrier in Donetsk on August 24. Ukraine has recently retaken control of much of its eastern territory bordering Russia, but fierce fighting for the rebel-held cities of Donetsk and Luhansk persists. 
People look at damaged Ukrainian military equipment in Donetsk on August 24. 
The first trucks of a Russian aid convoy roll on the main road to Luhansk in eastern Ukraine on Friday, August 22. The head of Ukraine's security service called the convoy a "direct invasion" under the guise of humanitarian aid since it entered the country without Red Cross monitors. 
A pro-Russian rebel holds shrapnel from a rocket after shelling in Donetsk, Ukraine, on August 22. 
Residents sit in a makeshift bomb shelter during a shelling in Makiyivka, Ukraine, on Wednesday, August 20. 
Dogs play together as a Russian convoy carrying aid supplies stops at a border control point with Ukraine, in the Russian town of Donetsk, in the Rostov-on-Don region on August 20. 
Ukrainian forces take their position not far from Luhansk, Ukraine, on August 20. Ukrainian troops made a significant push into rebel-held territory in eastern Ukraine, claiming control over a large part of Luhansk and encircling the largest rebel-held city, Donetsk, in fighting that has left at least 43 dead. 
Clouds of smoke are on the horizon as Ukrainian forces and pro-Russian rebels clash in Yasynuvata, a suburb of Donetsk, Ukraine, on Tuesday, August 19. 
An Ukrainian helicopter flies near Kramatorsk, Ukraine, on August 19. 
Ukrainian soldiers load a missile during fighting with pro-Russian rebels Monday, August 18, near Luhansk, Ukraine. 
Boys play at a refugee camp, set up by the Russian Emergencies Ministry, near the Russian-Ukrainian border on August 18. 
Ukrainian soldiers carry weapons at a checkpoint near Debaltseve, Ukraine, on Saturday, August 16. 
Pro-Russian rebels greet each other as they pass near Krasnodon, Ukraine, on August 16. 
A fireman tries to extinguish a fire after shelling in Donetsk on August 16. 
Ukrainian border guards patrol near Novoazovsk, Ukraine, on Friday, August 15. 
Trucks of a Russian humanitarian convoy are parked in a field outside the town of Kamensk-Shakhtinsky, in the Rostov region of Russia about 20 miles from the Ukraine border, on August 15. Ukrainian officials were preparing to inspect the convoy, which was bound for the conflict-torn east. 
A truck driver from the convoy jumps out of a trailer on August 15. The Ukrainian government had expressed fears that the convoy was a large-scale effort to smuggle supplies or troops to pro-Russian rebels. 
A tank belonging to pro-Russian rebels moves along a street in Donetsk on August 15. 
A Ukrainian soldier walks past a line of self-propelled guns as a column of military vehicles prepares to head to the front line near Illovaisk, Ukraine, on Thursday, August 14. 
A Ukrainian soldier prepares a mortar at a position near Illovaisk on August 14. 
A man inspects damage at his house after a shelling in Donetsk on August 14. 
A convoy of trucks, which Moscow said was carrying relief goods for war-weary civilians, moves from Voronezh, Russia, toward Rostov-on-Don, Russia, on August 14. 
Pro-Russian rebels on the outskirts of Donetsk stand at a checkpoint near a bullet-riddled bus on Wednesday, August 13. 
A pro-Russian rebel inspects damage after shelling in Donetsk on Thursday, August 7. 
Smoke billows from a Ukrainian fighter jet crash near the village of Zhdanivka, Ukraine, on August 7. 
Residents of eastern Ukraine cry in a hospital basement being used as a bomb shelter August 7 in Donetsk. 
Ordnance from a Ukrainian rocket launcher shoots toward a pro-Russian militant position in the Donetsk region on August 7. 
Relatives of Ukrainian military member Kyril Andrienko, who died in combat in eastern Ukraine, gather during his funeral in Lviv, Ukraine, on August 7. 
Refugees from southeastern Ukraine wait at a refugee camp in Donetsk on Wednesday, August 6. 
A pro-Russian rebel adjusts his weapon in Donetsk on August 6. 
Men walk past a bomb crater in Donetsk on August 6. 
A man steps out of his car as Ukrainian soldiers inspect the vehicle at a checkpoint in Debaltseve on August 6. 
Ukrainian servicemen sit on a bus near Slovyansk, Ukraine, on Tuesday, August 5. 
A pro-Russian separatist guards a road as Australian, Malaysian and Dutch investigators prepare to examine the crash site of Malaysia Airlines Flight 17 near the village of Rossipne, Ukraine, on August 5. U.S. and Ukrainian officials allege that a Russian-made missile shot down the plane from rebel-held territory, killing all 298 people on board. Russia and the rebel fighters deny involvement. 
Rescue workers carry the body of a woman who was killed during a bomb shelling in Donetsk on August 5. 
A boy stands in a hallway of a refugee hostel run by pro-Russian rebels in Donetsk on Monday, August 4. 
Ukrainian servicemen from the Donbass volunteer battalion clean their guns Sunday, August 3, in Popasna, Ukraine. 
Ukrainian soldiers fire shells toward rebel positions near Pervomaysk, Ukraine, on Saturday, August 2. 
Ukrainian troops patrol near the village of Novoselovka on Thursday, July 31. 
A woman says goodbye to her mother as she flees her home in Shakhtersk, Ukraine, on Tuesday, July 29. See more photos of the crisis from earlier this year Photos: Crisis in Ukraine 
Russia denies invasion of Ukraine U.S. President Barack Obama has opposed supplying Ukraine with weapons in the past, but Menendez said he believes the Obama administration should re-examine what he calls changed circumstances there.
"I think the European Union, NATO, as well as the United States has to consider this is dramatically different and we have to give the Ukrainians the fighting chance to defend themselves. I'm not suggesting U.S. troops here or NATO troops here ... but I am suggesting that the Ukrainians have the wherewithal to fight for themselves. "
Poroshenko warns of looming war with Russia
Ukrainian President Petro Poroshenko warned Saturday that his nation's crisis with Russia has worsened in recent days and is inching closer to a "full-scale war."
"I think we are very close to the point of no return," Poroshenko said following an EU summit in Brussels, Belgium. "The point of no return is full-scale war, which already happened in the territory controlled by separatists and where -- instead of separatists -- there are regular Russian troops."
Poroshenko said thousands of foreign troops and hundreds of foreign tanks are now on Ukrainian territory.
Russia has repeatedly denied supporting the rebels or sending troops over the border. But the West has rejected Moscow's denials.
European Council President Herman Van Rompuy, speaking at a meeting in Brussels, said new sanctions are in the works for Russia.
"The European Council says that it stands ready to take further significant steps in the light of the evolution of the situation on the ground and it requests the commission to urgently undertake preparatory work ... and present proposals for consideration within a week," Rompuy said.
As condemnation from the West escalated, Putin was defiant.
Moscow doesn't want or intend to wade into any "large-scale conflicts," Putin said, according to state-run ITAR-Tass. A few breaths later, he said Russia is "strengthening our nuclear deterrence forces and our armed forces," making them more efficient and modernized.
"I want to remind you that Russia is one of the most powerful nuclear nations," Putin said. "This is a reality, not just words."
Russian troops in Ukraine 
Russia's message on Ukraine 
Judah: 'Putin created a monster' Ukrainian port city fortifies defenses
The Ukrainian military worked to fortify the port city of Mariupol after Russian intelligence groups were observed there, said Col. Andriy Lysenko, a spokesman for the Ukrainian National Defense and Security Council.
A CNN team saw Ukrainian forces strengthening defensive positions on the eastern outskirts of the city, reinforcing checkpoints and digging trenches along roads leading toward the Russian border.
Beyond these checkpoints, the team found a small advance detachment of Ukrainian troops on the main road about halfway between Mariupol and Novoazovsk.
Months of violence leads to thousands of deaths
Russia annexed Ukraine's Crimean Peninsula in March, following the ouster of pro-Moscow President Viktor Yanukovych the previous month.
Violence broke out in the eastern Donetsk and Luhansk regions in April, as separatist leaders declared independence from the government in Kiev. Since mid-April, the conflict between the pro-Russia rebels and the Ukrainian military has cost more than 2,500 lives, according to the United Nations.
CNN's Tim Lister reported from Mariupol and Faith Karimi wrote and reported in Atlanta. CNN's Barbara Starr and Greg Botelho and journalist Victoria Butenko in Kiev contributed to this report.
