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
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From left, Moscow-appointed head of Kherson region Vladimir Saldo, Moscow-appointed head of Zaporizhzhia region Yevgeny Balitsky, Russian President Vladimir Putin, leader of the Donetsk People's Republic Denis Pushilin, and leader of the Luhansk People's Republic Leonid Pasechnik during a ceremony to sign an annexation agreement with Russia in Moscow. AP -
Russians gather for a celebration after the ceremony in central Moscow. EPA -
People gather near a screen showing Mr Putin during a broadcast of the ceremony. Reuters -
Mr Putin speaks during the ceremony to sign the treaties for the new territories' accession to Russia at the Grand Kremlin Palace in Moscow. EPA -
Law enforcement officers stand guard as people walk towards Red Square to attend the ceremony marking the annexation of the Russian-controlled territories. Reuters -
Participants listen to Mr Putin at the ceremony in the Georgievsky Hall of the Great Kremlin Palace in Moscow. Reuters -
Honour guards attend the ceremony. Reuters -
Russian Defence Minister Sergey Shoigu arrives at the ceremony. AP -
People walk towards Red Square in Moscow. Reuters -
People gather in front of screens located near the Kremlin before the live broadcast of the ceremony. Reuters
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.
