President Biden delivered a striking warning on Thursday night that recent threats from President Vladimir V. Putin of Russia could devolve into a nuclear conflict, telling supporters at a fundraiser in New York City that the risk of atomic war had not been so high since the 1962 Cuban Missile Crisis.

“We have not faced the prospect of Armageddon since Kennedy and the Cuban Missile Crisis,” Biden told a crowd at the second of two fundraisers he attended on Thursday evening.

“We are trying to figure out: What is Putin’s off-ramp?” Biden said, adding: “Where does he find a way out? Where does he find himself where hedoes not only lose face but significant power?”Biden’s references to Armageddon were highly unusual for any American President. Since the Cuban Missile Crisis, 60 years ago this month, occupants of the Oval Office have rarely spoken in such grim tones about the possible use of nuclear weapons, much less talked openly about “off ramps”.

The President’s warnings, delivered bluntly to a group of Democratic donors rather than in a more formal setting, came as analysts in Washington have been debating whether Putin might resort to tactical nuclear weapons to counter his mounting military losses in Ukraine.

In a speech last week, Putin raised the spectre of using nuclear weapons to hold on to his territorial gains, which Ukraine’s powerful counter-offensives have begun to erode. Putin said he would use “all available means” to defend Russian territory. The atomic bombs the US dropped on Japan in 1945, Putin said, had “created a precedent”.