Russian threats to use nuclear weapons are the most serious such warnings since the Cuban Missile Crisis, said US president Joe Biden.
Biden warned his Russian counterpart Vladimir Putin was "not joking" when he talks about the potential use of tactical nuclear weapons or biological or chemical weapons.
He added that Kremlin-backed forces were "underperforming" and said the US was trying to figure out Putin's "off-ramp" from the war.
"For the first time since the Cuban Missile Crisis, we have a direct threat to the use of nuclear weapons, if in fact things continue down the path they'd been going," Biden told Democratic donors in New York on Thursday.
"We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis," he said.
In the 1962 crisis, the United States under President John Kennedy and the Soviet Union under its leader, Nikita Khrushchev, came close to the use of nuclear weapons over the presence of Soviet missiles in Cuba.
"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 said.
Putin, who marks his 70th birthday on Friday, has warned he would use all means necessary, including Russia's nuclear arsenal, to protest Russian soil.
It comes after Moscow annexed four Ukrainian regions, which it says are now part of its territory.
In remarks to Australia's Lowy Institute, Zelenskyy said NATO should launch preventive strikes on Russia to preclude its use of nuclear weapons.
The Kremlin denounced Zelenskyy's comments as "an appeal to start yet another world war with unpredictable, monstrous consequences", according to RIA news agency.
A Ukrainian counter-offensive has pushed Russian forces into further retreating and
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