Biden rebukes Putin after new Ukraine escalation

US President Joe Biden tore into Vladimir Putin as he addressed the United Nations just hours after the Russian leader dramatically escalated his seven-month war in Ukraine by calling up his country's military reservists.

Biden accused Putin of "shamelessly" violating the UN Charter and castigated him over a veiled threat to use nuclear weapons -- after Putin said his promise to use all military means in Ukraine was "no bluff."

"Russia has shamelessly violated the core tenets of the United Nations Charter," Biden said as he addressed the UN General Assembly, warning that "a nuclear war cannot be won and must never be fought."

Russian forces have attacked Ukrainian schools, railway stations and hospitals during a war that Biden said was aimed at "extinguishing Ukraine's right to exist as a state."

Putin's mobilisation call came as Moscow-held regions of Ukraine prepare to hold annexation referendums this week, ramping up the stakes in the conflict by allowing Moscow to accuse Ukraine of attacking Russian territory.

Four Russian-occupied regions of Ukraine -- Donetsk and Lugansk in the east and Kherson and Zaporizhzhia in the south -- said that they would hold the votes over five days beginning Friday.

In a pre-recorded address to the nation, Putin accused the West of trying to "destroy" his country through its backing of Kyiv. Russia needed to support those in Ukraine who wanted to "determine their own future", he said.