UNITED NATIONS - United States President Joe Biden took centre stage at the United Nations to rally international support for Ukraine following Russia’s military escalation and Mr Vladimir Putin’s threat to use nuclear weapons.

“Russia has shamelessly violated the core tenets of the United Nations Charter,” Mr Biden said as he addressed the UN General Assembly.

Russian forces have attacked Ukrainian schools, railway stations and hospitals, part of Moscow’s aim of “extinguishing Ukraine’s right to exist as a state”, he added.

Mr Biden's response comes a day after France’s Emmanuel Macron and Germany’s Olaf Scholz decried Mr Putin’s aggression, both accusing him of “imperialism”.

Mr Putin has not attended UN since 2015, the year he sent military aircraft into Syria.

On Wednesday, the Russian leader 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 Russia’s looming defeat.

Flights out of Russia quickly sold out following the announcement of the country’s first military mobilisation since World War II, a dramatic reversal after months in which Moscow had insisted its operation was “going to plan”.

Russian Defence Secretary Sergei Shoigu said 300,000 people would be mobilised out of a pool of 25 million. The contracts of professional soldiers would be extended indefinitely.

In his televised address, Mr Putin effectively announced plans to annex four Ukrainian regions, saying Moscow would facilitate referendums in Ukraine’s Luhansk, Donetsk, Zaporizhzhia and Kherson regions on joining Russia. A day earlier, Russian-installed officials in the four regions announced plans for such votes this week, which Western countries denounced as shams.

Mr Putin said, with no evidence, that officials in Nato states had threatened to use nuclear weapons against Russia but that Russia “also has various means of destruction”.

“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,” he said.

Ukrainian President Volodymyr Zelensky said he thought Mr Putin would be unlikely to use nuclear weapons but that the threat showed why it was important to stand up to him.

“I don’t believe that he will use these weapons. I don’t think the world will allow him to use these weapons,” Mr Zelensky said in remarks reported by Germany’s Bild newspaper.

“Tomorrow Putin can say: ‘Apart from Ukraine, we also want a part of Poland, otherwise we will use nuclear weapons.’ We cannot make these compromises,” Mr Zelensky said.

In an interview with Reuters, Nato Secretary-General Jens Stoltenberg denounced Mr Putin’s threat to use nuclear weapons as “dangerous and reckless rhetoric”. The mobilisation plan demonstrated “that the war is not going according to his plans”, he added.

As for any potential Russian use of nuclear arms, “We will make sure that there is no misunderstanding in Moscow about exactly how we will react,” Mr Stoltenberg said.

“The most important thing is to prevent that from happening and that is why we have been so clear in our communications with Russia about the unprecedented consequences,” he said. Mr Putin’s announcement came after weeks in which Russia’s invasion force was routed in northeastern Ukraine, with thousands of Russian troops fleeing frontline positions in the biggest shift in momentum since the early weeks of the war which was launched on Feb 24.

Ukrainian forces have captured some of the main supply routes that had served Russia’s front line in the east, and say they are now poised to push deeper into territory that Moscow had captured over months of heavy fighting.

“No amount of threats and propaganda can hide the fact that Ukraine is winning this war, the international community are united and Russia is becoming a global pariah,” said British Defence Secretary Ben Wallace.

Australia and New Zealand said Putin's threats to use nuclear weapons to defend Russia were “unthinkable” and exposed his justification for the war as untrue.

“These threats are unthinkable and they are irresponsible.  His claims of defending Russia’s territorial integrity are untrue. No sham referendum will make them true,” Australia’s Foreign Affairs Minister Penny Wong said in New York, where she is attending the United Nations General Assembly.  

“Russia alone is responsible for this illegal and immoral war, and peace must first lie with Russia withdrawing from Ukrainian territory,” she added.  

New Zealand’s Prime Minister Jacinda Ardern strongly condemned Russia’s escalation, saying Putin’s claim he could use additional weapons “flies in the face of the lie that they have told that they are there to liberate others”.  

“This highlights the falsehood around this war,” she told media in New York, where she is attending the United Nations General Assembly.

“What is happening in Ukraine is illegal, it’s immoral, it’s causing the loss of civilian life and that loss could extend if, as Putin has claimed, he broadens the types of weapons he uses in this war,” she added. 

Canadian Prime Minister Justin Trudeau said Putin's move showed that the Ukraine invasion is failing, as he condemned Moscow’s announcement as unacceptable.  

“Canada condemns Putin’s irresponsible escalation of the war, his partial military mobilization, his nuclear threats, as well as Russia’s rushed referendums to try to annex parts of Ukraine are unacceptable,” Trudeau told reporters in New York, where he is attending the United Nations General Assembly.  

“Putin’s behaviour only goes to show that his invasion is failing,” Trudeau said.

Several Western military experts said drafting in hundreds of thousands of new troops would take months, do little to slow Russia’s losses, and could even make matters worse by drawing resources away from the battlefield to train and equip recruits.

“Jaw-dropping. A new sign of RU weakness,” tweeted Mr Mark Hertling, a former commander of US ground forces in Europe.

“Placing ‘newbies’ on a front line that has been mauled, has low morale & who don’t want to be (there) portends more RU disaster,” he said.

BLOOMBERG, AFP, REUTERS