US President Joe Biden has said the world is facing the biggest threat of nuclear “Armageddon” since the days of President Kennedy and the Cuban Missile Crisis.

Speaking at a Democratic Party fundraiser in Washington, his ‘off camera’ comments were overheard by journalists from the White House pool.

Describing Russian President Vladimir Putin, President Biden said he had “got to know the guy pretty well”.

“He’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.”

And he made clear his concerns that if such weapons were deployed it would quickly escalate into world nuclear conflict.

“First time since the Cuban missile crisis, we have a direct threat of the use nuclear weapon if in fact things continue down the path they are going,” he said.

During the 1962 crisis, the United States under President John Kennedy and the Soviet Union under Nikita Kruschev came close to using nuclear weapons over the presence of Soviet missiles in Cuba.

More from World

Mr Biden said he and US officials were searching for a diplomatic off-ramp.

“I’m 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 that he does not not only lose face but lose significant power within Russia?

“We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis.”

He added: “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.

“I didn’t realise how much serious damage the previous administration did to our foreign policy.”

US officials have warned for the last few months about the prospect of Russia using weapons of mass destruction in Ukraine after numerous strategic setbacks on the battlefield.

Mr Putin has repeatedly alluded to using his country’s vast nuclear arsenal.

Last month, when he announced plans to conscript Russian men to serve in Ukraine, he said: “I want to remind you that our country also has various means of destruction … and when the territorial integrity of our country is threatened, to protect Russia and our people, we will certainly use all the means at our disposal.”

With a lingering stare at the camera, he added: “It’s not a bluff.”

On Thursday Ukrainian President Volodymyr Zelenskyy said Mr Putin understood the “world will never forgive” a Russian nuclear strike.

“He understands that after the use of nuclear weapons he would be unable any more to preserve, so to speak, his life, and I’m confident of that,” Mr Zelenskyy said.