"We have not seen any reason to adjust our own strategic nuclear posture, nor do we have indications that Russia is preparing imminently to use nuclear weapons," Press Secretary Karine Jean-Pierre told reporters aboard Air Force One.

Asked if Biden's alarming comment -- made late Thursday while criticizing Russian President Vladimir Putin -- reflected new intelligence, she said "no."

"The president was speaking about concerns about Putin's threats to use nuclear weapons," she said.

Asked about Biden's assertion on Thursday that the United States was trying to find an "off ramp" for Putin to deescalate in his invasion of Ukraine before he resorts to weapons of mass destruction, Jean-Pierre said the US position has not changed.

"There is only one country responsible for this war," she said. "And that's Russia. They started this conflict and Mr Putin has the ability to stop this conflict today."

Biden's comments on Thursday were unusually direct about the dangers from Putin's thinly veiled threats to use nuclear weapons to assist Russia's faltering attempt to take over swaths of Ukraine.

Referring to the nuclear standoff 60 years ago triggered by the Soviet Union stationing missiles in Cuba, within easy range of the United States, Biden said that for the "first time since the Cuban missile crisis, we have a direct threat from the use of nuclear weapons, if in fact things continue down the path they are going."

"We have not faced the prospect of Armageddon since" then, Biden said.

Jean-Pierre said that "if the Cuban missile crisis has taught us anything, it is the value of reducing nuclear risk and not brandishing that."