Other Western leaders gathered in New York vowed not to recognise the sham referendums and characterised Putin’s escalation as a sign of weakness.
Jens Stoltenberg, the Nato secretary-general, said in response to Putin’s threats: “The answer is not to step down and to stop supporting Ukraine. The answer, if anything, is to step up and further support Ukraine.
“We will make sure that there is no misunderstanding in Moscow about the seriousness of using nuclear weapons.
“We have been so clear in our communications with Russia about the unprecedented consequences, about the fact that the nuclear war cannot be won by Russia.”
He said the only way to end the war was to “prove that President Putin will not win on the battlefield”.
Putin’s escalation followed mounting casualties and setbacks for Russian forces as they were driven back by a Ukrainian counter-offensive this month.
Following his mobilisation announcement, young Russian men began trying to leave the country.
Plane tickets to countries that do not require a visa, like Turkey and Belarus, quickly sold out.
There were protests in dozens of cities across Russia, some turning violent.
In Moscow, protesters chanted: “Send Putin to the trenches.” In the city of Novosibirsk, a young Russian man was filmed yelling: “I don’t want to die for Putin.”
As of Wednesday night, 700 people had been arrested by Russian authorities in more than 35 cities.