"We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis" in 1962, Biden said at a Democratic Party fundraising event in New York, referring to former US president John F Kennedy.

Putin is "not joking" when he threatens to use nuclear weapons to pursue his invasion of Ukraine, Biden said.

The president made his unusually strong comments about the risks created by Putin's nuclear threats while speaking to party supporters at an event hosted in the Manhattan home of James Murdoch, son of newspaper mogul Rupert Murdoch.

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're trying to figure out what is Putin's off-ramp," Biden said.

Putin has made thinly veiled threats to use nuclear weapons if he feels he has run out of options in his bid to seize swaths of Ukrainian territory in the face of stiff resistance by Western-back Kyiv.

Experts say these would most likely be relatively small, tactical strikes. But Biden warned such strikes in a limited area would still risk triggering a wider conflagration.

"We've got a guy I know fairly well," Biden said. Putin's "not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons, because his military is, you might say, significantly underperforming."

But "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.

The president's comments come as Ukraine has recaptured fresh territory from Russian troops, the latest in a series of Moscow defeats undermining the Kremlin's claim to have annexed around 20 percent of Ukraine.

But with Russian's once-vaunted military faltering, Washington and its Western allies have grown concerned about what Putin might be planning on the battlefield.

"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?"