President Joe Biden has warned Russia that the United States will not be intimidated by reckless threats after Vladimir Putin declared the annexation of four occupied regions of Ukraine.

 President Putin appeared to make a veiled threat to use nuclear weapons to defend those regions.

He said they would "forever" be Russian - but Ukraine vowed to liberate them.

Nato chief Jens Stoltenberg said the Russian move was "the most serious escalation since the start of the war".

In a speech in Moscow, the Russian leader claimed citizens in Ukraine's eastern Donetsk and Luhansk regions, as well as in Kherson and Zaporizhzhia in the south had voted to be "with their people, their motherland".

He was referring to so-called referendums held in the regions in recent days, but Ukraine and Western governments have condemned the votes as a sham.