The United States on Thursday denounced Russian President Vladimir Putin's warnings of a risk of nuclear war as "irresponsible," but said there was no sign of greater risk.

"It is not the first time we have seen irresponsible rhetoric from Vladimir Putin. It is no way for the leader of a nuclear-armed state to speak," State Department spokesman Matthew Miller told reporters.

"We have communicated in the past privately and directly with Russia about the consequences of the use of a nuclear weapon," he said.

But Miller added: "We do not have any sign that Russia is preparing to use a nuclear weapon."

Putin, in an address to the nation on Thursday, warned the West of "tragic consequences" for any country that sends troops into Ukraine to defend against Russia's invasion.

"Everything that the West comes up with creates the real threat of a conflict with the use of nuclear weapons, and thus the destruction of civilization," Putin said.

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