Speaking to party donors, Biden said Russian President Vladimir Putin was “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 challenged Russian nuclear doctrine, warning that the use of a lower-yield tactical weapon could quickly spiral out of control.
“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 added.
“We have not faced the prospect of Armageddon since Kennedy and the Cuban Missile Crisis.”
The 13-day crisis in 1962, when the US discovered the then Soviet Union had deployed nuclear missiles to Cuba, is thought to be the closest the world has ever come to nuclear annihilation.
US officials have been warning for months of the prospect that Moscow could use weapons of mass destruction in Ukraine as it faces a series of strategic setbacks on the battlefield and Ukraine retakes more of its lands in the south and east of the country.
Biden’s remarks are the starkest yet made by anyone in the US government about the nuclear stakes, but the president also stressed officials were trying to “figure out” how Putin might be able to extricate himself from the situation.
“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?”
“We will defend our land with all the powers and means at our disposal,” he said.
The annexations followed hastily-organised referendums in the four areas that Ukraine and its allies condemned as a “sham” and illegal.
United Nations chief Antonio Guterres described the plan as a “dangerous escalation“.
Ukraine President Volodymyr Zelenskyy said earlier on Thursday that Putin understood that 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,” Zelenskyy said.