Joe Biden has issued a stark warning to Vladimir Putin that Russia would not win a nuclear war.
The US president, addressing the United Nations General Assembly in New York, accused Putin of making “overt, reckless and irresponsible” nuclear threats.
He said such a war must “never be fought” and that Putin’s actions in Ukraine, and his threats, should make the world’s “blood run cold”.
Mr Biden also accused China of conducting a “concerning” nuclear build-up and vowed that the US would never allow Iran to develop atomic weapons.
Emphasising each word slowly, Mr Biden said: “A nuclear war can not be won and must never be fought.”
He said: “No matter what else is happening in the world, the United States is ready to pursue critical arms control measures.”
Mr Biden’s comments came hours after Putin said he was “not bluffing” on the potential use of nuclear weapons.
The Russian leader also ordered a partial mobilisation of 300,000 reservists in a desperate attempt to turn the tide of the war in Ukraine.
Referendums are set to be held in four occupied areas of Ukraine later this week, and Russia is expected to declare that they have been annexed.
In a televised address to his nation, Putin made a clear reference to nuclear weapons, saying he would “use all means” to defend Russian territory. He added: “This is not a bluff.”
Mr Biden responded that the Russian leader had “shamelessly violated the core tenets” of the United Nations Charter.
He said Putin had made “overt nuclear threats against Ukraine in reckless disregard for the responsibilities” of Russia as a signatory of the Treaty on the Non-Proliferation of Nuclear Weapons.
Mr Biden said: “They [Russia] are making irresponsible threats to use nuclear weapons.
“Now, Russia is calling up more soldiers to join the fight and the Kremlin is organising sham referendums. The world should see these outrageous acts for what they are. No one threatened Russia and no one other than Russia sought conflict.
“This war is about extinguishing Ukraine’s right to exist as a state and Ukraine’s right to exist as a people.
“Wherever you live, whatever you believe, that should make your blood run cold.”
Mr Biden added that it was a “brutal needless war, a war chosen by one man, to be very blunt”.
The US president’s speech was hastily re-written after Putin announced his biggest escalation since invading Ukraine on Feb 24.
Mr Biden said: “The United States wants this war to end on just terms. You cannot seize a nation’s territory by force. The only country standing in the way of that is Russia.
“We will stand in solidarity with Ukraine. We will stand in solidarity against Russia’s aggression. Period.”
He said the Russian threats were not the only “disturbing trend” in relation to nuclear weapons.
Mr Biden said: “China is conducting a concerning nuclear build-up without any transparency.”
He added: “The United States is clear – we will not allow Iran to acquire a nuclear weapon.
“I continue to believe diplomacy is the best way to achieve this outcome. We cannot let the world now slide backwards.”
Mr Biden also said the US “stands with the brave women of Iran” during growing protests over the death of a young woman arrested by the country’s morality police.
He said: “Today we stand with the brave citizens and the brave women of Iran who right now are demonstrating to secure their basic rights.”