Putin threatened the use of nuclear weapons after he announced a partial mobilization in September.
Biden said at a fundraiser in New York City that he's taking the threats seriously.
Experts raised alarm at Putin's threats but have said the chances of nuclear warfare remain low.
President Joe Biden said Thursday that the prospect of Russian President Vladimir Putin using nuclear weapons in the war with Ukraine is a threat he's taking very seriously.
Putin is "not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons because his military is… underperforming," Biden said while speaking at a fundraiser in New York City, according to Wall Street Journal reporter Tarini Parti and The Associated Press.
"We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis," the president said, explaining that he didn't think there was a way to use tactical nuclear weapons and not have it result in Armageddon.
Putin threatened the use of nuclear weapons when he announced partial military mobilization and escalated the war on September 21.
"The territorial integrity of our motherland, our independence, and freedom will be secured, I repeat with all the means we have," Putin said in his address in September. "Those who try to blackmail us with nuclear weapons should know that the prevailing winds can turn in their direction."
Analysts have raised alarms at Putin's recent statements, but maintained the chances of it actually happening still remain low.
Hans Kristensen, a director of the Nuclear Information Project at the Federation of American Scientists, told Insider that the threats reflect a change in Putin's rhetoric around nuclear warfare, but that it's still yet to be determined if it means a change in "actual planning."
People should still be "extraordinarily concerned," Kristensen said, and that "very deliberate efforts" should be taken to convince Russia of the consequences of taking the conflict to the next level.
George Barros, a military analyst with the Institute for the Study of War, also told Insider that while the possibility of Putin using nuclear weapons should not be ruled out, that "risk is low."
"The problem for the Russians is that in order to properly exploit the conditions that are created by a tactical nuclear weapon, you need to have forces that are cohesive, coherent, and with a high level of morale," Barros said, adding: "I don't think the conventional Russian military, at this point, has the capability or the morale to be able to do that."
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