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 New York, his 'off camera' comments were overheard by journalists from the White House pool.
It comes as Russian officials warned of the possibility of using tactical nuclear weapons after suffering huge military setbacks in the invasion of Ukraine.
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 the 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.
"For the first time since the Cuban missile crisis, we have a direct threat of the use of nuclear weapons if in fact things continue down the path they are going," he said.
"I'm trying to figure out what is Putin's off-ramp?" he said. "Where does he find a way out? Where does he find himself in a position that he does 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."
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.
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 anymore to preserve, so to speak, his life, and I'm confident of that," Mr Zelenskyy said.