Ukrainian President Volodymyr Zelenskyy said on Friday that Ukraine was applying to join Nato and seeking what he called an accelerated accession.
Mr Zelenskyy's surprise announcement came shortly after Russian President Vladimir Putin declared the annexation of four occupied regions of Ukraine.
He said his country's seven-month resistance to the Russian invasion had shown that Ukrainian forces were up to Nato standards and that allies were already working in concert with Ukraine.
"De facto, we have already made our way to Nato," Mr Zelenskyy said after meeting defence and security chiefs on Friday. "Today, Ukraine is applying to make it de jure.
"We are taking a decisive step by signing Ukraine's application for accelerated accession to Nato."
Nato chief Jens Stoltenberg seemed noncommittal about Ukraine's move in his remarks to media on Friday.
He told reporters that Nato members “support Ukraine’s right to choose its own path, to decide what kind of security arrangements it wants to be part of” while their current priorities are “providing immediate support to Ukraine".
Ukraine's membership would require the approval of all 30 Nato members.
The signing ceremony in Kyiv mirrored the one in an ornate Kremlin hall only moments earlier, in which Mr Putin declared Donetsk, Luhansk, Zaporizhzhia and Kherson to be part of Russia.
Vladimir Putin annexes four regions of Ukraine - in pictures
Western powers dismissed the annexations as illegal and the referendums leading to them as a sham. US President Joe Biden announced new sanctions and said Russia was "showing its contempt for peaceful nations everywhere".
Ukraine's long-standing ambition to join Nato is a prime grievance for Russia and was one of the reasons behind the invasion in February.
In early peace talks in March, Ukraine signalled was willing to pursue alternative security arrangements. But the negotiations floundered and Mr Zelenskyy on Friday suggested no more talks were possible with Mr Putin.
"We are ready for a dialogue with Russia, but with a different Russian president," he said.
Nato membership would bring Ukraine under the umbrella of the alliance's Article 5 mutual defence guarantee, a prospect that allies have previously baulked at.