US President Joe Biden said Russian President Vladimir Putin's threat to use nuclear weapons threatens to bring about the biggest such risk since the Cuban Missile Crisis, adding Washington was "trying to figure out" Putin's off-ramp.

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 Russia.

"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 Party 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 said.

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. (Reuters)