At a Democratic fundraiser hosted by James Murdoch, Biden characterized Russian Czar Vladimir Putin as "a guy I know fairly well" -- and added, Putin was "not joking when he talks about the use of tactical nuclear weapons or biological or chemical weapons."

Biden went on to say, "We have not faced the prospect of Armageddon since Kennedy and the Cuban Missile Crisis." He also implied the threat from Putin is tangible "because his military is -- you might say -- significantly underperforming."

The Cuban Missile Crisis was a 13-day showdown in 1962 born out of the Soviet Union's deployment of nuclear weapons in Cuba ... prompting the Kennedy administration to launch a naval blockade of the island.

As far as the public knows, it remains the closest the U.S ever came to a nuclear war.

Biden says he's still "trying to figure" out Putin's "off-ramp" in Ukraine. Where does he find a way out?" Biden asked. "Where does he find himself in a position that he does not not only lose face but lose significant power within Russia?"

Despite Biden's chilling warning, U.S. officials have NOT seen Russia change its nuclear posture to launch an imminent attack.