Russia has formally annexed four occupied regions of Ukraine, in a move sparking international condemnation.

President Vladimir Putin signed "accession treaties" with the regions' Moscow-installed leaders at a ceremony in the Kremlin's opulent St George's Hall on Friday.

Watched by members of the political elite, he declared the regions would "forever" be part of Russia.

Kyiv reacted by launching a new, fast-track bid to join the Nato alliance.

Flanked by his prime minister and the speaker of parliament, Ukrainian President Volodymyr Zelensky said Ukraine had long been a "de-facto" member of the security bloc, and accused Moscow of redrawing borders "using murder, blackmail, mistreatment and lies".

"We are taking our decisive step by signing Ukraine's application for accelerated accession to Nato," Mr Zelensky said, speaking from the centre of Kyiv.

Nato Secretary-General Jens Stoltenberg was reluctant to be drawn on the bid, saying the decision rested with the bloc's 30 members. But he condemned Moscow's annexation of Ukrainian territory, calling the move "the most serious escalation since the start of the war".