US Secretary of State John Kerry said on Wednesday that Russia must be "very careful" in the decisions it makes on Ukraine, and that the country's President should remember "this is not Rocky IV".
"I think Russia needs to be very careful in the judgments that it makes going forward here," he told US TV network NBC News.
"We are not looking for confrontation. But we are making it clear that every country should respect the territorial integrity here, the sovereignty of Ukraine.
"Russia has said it would do that and we think it's important that Russia keeps its word," he added.
Mr Kerry said that Russian President Vladimir Putin should "listen carefully to Ukrainians who have voiced their desire for change," and reiterated that the US does not view its relationship with Russia as a "sort of continuation of the Cold War."