US President Joe Biden accused Russia of making "reckless" and "irresponsible" threats to use nuclear weapons and said Moscow had violated the core tenets of United Nations membership by invading Ukraine.
Speaking at the United Nations General Assembly in New York, Biden slammed Russian President Vladimir Putin for starting an unprovoked war that some 40 UN members were helping Ukraine fight through funding and weapons.
Earlier on Wednesday, Putin ordered a Russian mobilisation to fight in Ukraine and made a thinly veiled threat to use nuclear weapons, in what NATO called a "reckless" act of desperation in the face of a looming Russian defeat.
Biden echoed that sentiment.
"Again, just today, President Putin has made overt nuclear threats against Europe, in a reckless disregard for the responsibilities of the nonproliferation regime," Biden said.
"A nuclear war cannot be won and must never be fought," he said.
Biden said no one had threatened Russia, despite its claims to the contrary, and that only Russia had sought conflict, and he used the U.N. setting to underscore his view that Moscow had violated the body's values.
"A permanent member of the United Nations Security Council invaded its neighbor, attempted to erase a sovereign state from the map. Russia has shamelessly violated the core tenets of the United Nations Charter," Biden said.
"This war is about extinguishing Ukraine's right to exist as a state, plain and simple, and Ukraine's right to exist as a people. Wherever you are, wherever you live, whatever you believe, that should ... make your blood run cold."
Russia's mission to the United Nations did not immediately respond to a request for comment on Biden's remarks. While Russian Foreign Minister Sergei Lavrov was in New York for the UN gathering, a deputy Russian UN ambassador was in the chamber for Biden's speech.
Putin's "partial mobilisation" decree did not spell out who would be called up. Defence Secretary Sergei Shoigu said 300,000 people would be mobilised from a pool of 25 million. Contracts of professional troops would be extended indefinitely.
Flights out of Russia quickly sold out following Putin's announcement, while jailed opposition leader Alexei Navalny called for mass demonstrations against the mobilisation.
A monitoring group said more than 100 people were arrested in protests in the hours after Putin's speech.
Putin effectively announced plans to annex four Ukrainian provinces, saying Moscow would assist with referendums on joining Ukraine's Luhansk, Donetsk, Zaporizhzhia and Kherson regions to Russia, and implement the results.
A day earlier, Russian-installed officials in the four regions announced plans for such votes from later this week, which Western countries denounced as sham plebiscites.
Putin said, offering no evidence, that officials in NATO states had threatened to use nuclear weapons against Russia, but should know that "the weathervane can turn towards them".
Russia "also has various means of destruction", he said. "When the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. It's not a bluff."
- Reuters