Joe Biden said that the world is facing its biggest threat of a nuclear "Armageddon" since the Cuban missile crisis due to the danger of Vladimir Putin using the weapons over Ukraine.
The US President has said that Washington is still "trying to figure out" where Putin can find a way out from the crisis.
The White House has said repeatedly that it has seen no indication that Russia is preparing to use nuclear weapons despite what it calls Putin's "nuclear sabre-rattling."
But Biden on Thursday made clear he was keeping a wary eye on Putin and how he might react as Ukraine's military makes gains against Russian invaders.
"For the first time since the Cuban Missile Crisis, we have a direct threat to the use of nuclear weapons, if in fact things continue down the path they'd been going," Biden told Democratic donors in New York.
He also said, "we have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis."
In the 1962 Cuban Missile Crisis, the United States under President John Kennedy and Soviet Union under its leader, Nikita Khrushchev, came close to the use of nuclear weapons over the presence of Soviet missiles in Cuba.
Putin, said Biden, is "not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons, because his military is, you might say, is significantly underperforming."
"I don't think there's any such thing as the ability to easily (use) a tactical nuclear weapon and not end up with Armageddon," Biden continued.
Russian forces have suffered heavy losses since their invasion of Ukraine began in February and are now retreating from the east and south of the country.
Biden said he and US officials are searching for a diplomatic 'off-ramp.'
"We're trying to figure out what is Putin's off-ramp... Where does he find a way out? Where does he find himself in a position he does not, not only lose face but lose significant power in Russia," Biden said.
Biden spoke at the New York home of businessman James Murdoch, turning to the son of conservative media mogul Rupert Murdoch to try to boost his party's chances in November 8 congressional elections.
The event at Murdoch's home was to benefit the Democratic Senatorial Campaign Committee, which is providing support for Democratic candidates for the Senate.
