From left, Moscow-appointed head of Kherson Region Vladimir Saldo, Moscow-appointed head of Zaporizhzhia region Yevgeny Balitsky, Russian President Vladimir Putin, Denis Pushilin, leader of self-proclaimed Donetsk People's Republic and Leonid Pasechnik, leader of self-proclaimed Luhansk People's Republic pose for a photo during a ceremony to sign the treaties for four regions of Ukraine to join Russia, at the Kremlin in Moscow, Sept. 30. AP-Yonhap

Russian President Vladimir Putin signed treaties Friday to illegally annex more occupied Ukrainian territory in a sharp escalation of his war. Ukraine's president countered with a surprise application to join the NATO military alliance.

Putin's land-grab and President Volodymyr Zelenskyy's signing of what he said is an ''accelerated'' NATO membership application sent the two leaders speeding faster on a collision course that is cranking up fears of a full-blown conflict between Russia and the West.

Putin vowed to protect newly annexed regions of Ukraine by ''all available means,'' a renewed nuclear-backed threat he made at a Kremlin signing ceremony where he also railed furiously against the West, accusing the United States and its allies of seeking Russia's destruction.

Zelenskyy then held his own signing ceremony in Kyiv, releasing video of him putting pen to papers he said were a formal NATO membership request.

Putin has repeatedly made clear that any prospect of Ukraine joining the military alliance is one of his red lines and cited it as a justification for his invasion, now in its eighth month, in Europe's biggest land war since World War II.

In his speech, Putin urged Ukraine to sit down for peace talks but insisted he won't discuss handing back occupied regions. Zelenskyy said there'd be no negotiations with Putin.

"We are ready for a dialogue with Russia, but with another president of Russia," the Ukrainian leader said.

At his signing ceremony in the Kremlin's ornate St. George's Hall, Putin accused the West of fueling the hostilities to turn Russia into a ''colony'' and a ''crowd of soulless slaves.'' The hardening of his position, in the conflict that has killed and wounded tens of thousands of people, further raised tensions already at levels unseen since the Cold War.

Global leaders, including those from the Group of Seven leading economies, responded with an avalanche of condemnation. The U.S. and the U.K. announced more sanctions.

U.S. President Joe Biden said of Putin's annexation of the Donetsk, Luhansk, Kherson and Zaporizhzhia regions: ''Make no mistake: These actions have no legitimacy.''

''America and its allies are not going to be intimidated by Putin and his reckless words and threats,'' Biden added, noting that the Russian leader ''can't seize his neighbor's territory and get away with it.''

The European Union said its 27 member states will never recognize the illegal referendums that Russia organized ''as a pretext for this further violation of Ukraine's independence, sovereignty and territorial integrity.''

People watch a large screen at the Kremlin in Moscow, as Russian President Vladimir Putin delivers a speech after a ceremony to sign the treaties for four regions of Ukraine to join Russia, during a meeting in Sevastopol, Crimea, Sept. 30. AP-Yonhap

Russia vetoed a U.N. Security Council resolution Friday that would have condemned the referendums, declared that they have no validity and urged all countries not to recognize the annexation. China, India, Brazil and Gabon abstained on the vote in the 15-member council.

NATO Secretary-General Jens Stoltenberg called it ''the largest attempted annexation of European territory by force since the Second World War.''

The war is at ''a pivotal moment,'' he said, and Putin's decision to annex more territory - Russia now claims sovereignty over 15% of Ukraine - marks ''the most serious escalation since the start of the war.'' Stoltenberg was noncommittal on Zelenskyy's fast-track NATO application, saying alliance leaders ''support Ukraine's right to choose its own path, to decide what kind of security arrangements it wants to be part of.''

Dmitry Medvedev, deputy head of Russia's Security Council, said Zelenskyy's move toward the military alliance amounts to ''begging NATO to accelerate the start of World War III.''

Zelenskyy vowed to keep fighting, defying Putin's warnings that Kyiv shouldn't try to recapture what it has lost.