President Volodymyr Zelenskyy said on Friday Ukraine was formally applying for fast-track membership of the NATO military alliance and that Kyiv was ready for talks with Moscow, but not with President Vladimir Putin.

The Ukrainian leader made his comments in a video which appeared intended as a forceful rebuttal to the Kremlin after Putin held a ceremony in Moscow to proclaim four partially occupied Ukrainian regions as annexed Russian land.

"We are taking our decisive step by signing Ukraine's application for accelerated accession to NATO," Zelenskyy said in a video on Telegram.

The video showed Zelenskyy announcing the decision and then signing a document flanked by his prime minister and the speaker of parliament.

"Ukraine is ready for negotiations, but with a different Russian president," Zelenskyy said.