WATCH: Durbin says Crimea isn't end of Putin's ambition

Sen. Dick Durbin (D-Ill.) went on NBC’s “Meet the Press” Sunday morning and said that Russia’s actions in Ukraine are “the single most serious act of aggression since the Cold War.”

Durbin went on to say that the U.S. and its Western allies need to band together to push back against 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.

He was joined by Sen. Jeff Flake (R-Ariz.), who like Durbin, is a member of the Senate Foreign Relations Committee and recently returned from Ukraine. Flake said U.S. action is necessary, but expressed concern if anything could be done 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.”