Sen. Dick DurbinDick DurbinDems don’t want to help GOP improve repeal bill Mnuchin: Trump administration examining online sales tax issue Senate Dem: We’re trying to block a recess appointment to replace Sessions MORE (D-Ill.) said Sunday that Russia’s actions surrounding Ukraine are “the single most serious act of aggression since the Cold War.”

The member of the Senate Foreign Relations Committee, just back from a trip to Ukraine, said the United States and its Western allies needed to push back against Russian President Vladimir Putin.

“Are we going to stand by and say this is acceptable conduct? Because this isn’t the end of his ambition,” he said on NBC’s “Meet the Press.”

Sen. Jeff Flake (R-Ariz.), another committee member who was part of the delegation, agreed that U.S. action was necessary, but was concerned about whether the West would be able to dissuade Putin from claiming Crimea.

“It’s going to be difficult. Let’s face it, Russia has always had a design on Crimea,” he said. “All you an do is increase the costs significantly and hope that they don’t move further into Ukraine.”

The Senate appears poised to pass legislation later this month that would provide aid to Ukraine and impose new Russian sanctions. But the House and some Senate Republicans have criticized the package for including reforms to the International Monetary Fund, slowing its progress.

Flake split from his GOP colleague on the committee by not assigning any blame to the White House for Putin’s actions.

Appearing on “Fox News Sunday,” Sen. Bob Corker (Tenn.), the top Republican on the panel, accused the administration of creating “an air of permissiveness” that emboldened Putin. Flake disagreed.

“I don’t think anything the president did or said lent itself to what Putin did here,” he said.

Their comments came as Crimea held a vote on whether to secede from Ukraine and potentially join Russia. The U.S. has called the election illegal and vowed not to recognize the result, which is widely expected to show public support for the Russian-speaking Ukrainian region to join Russia.

Durbin called the referendum “a lame excuse by Putin to invade Crimea,” noting the presence of Russian soldiers, sometimes not wearing identifiable uniforms, as creating an intimidating atmosphere for voters.

“This is a Soviet-style election. We know what the end is going to be,” he said.