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."
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."
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.