NATO accused Russian President Vladimir Putin on Friday of provoking "the most serious escalation" of the war in Ukraine since it began, but said he would not succeed in deterring the alliance from supporting Kyiv.

"We have the combination of the mobilization in Russia, combined with the reckless, dangerous nuclear rhetoric, and then today's illegal annexation, or attempt to annex parts of Ukraine," NATO Secretary General Jens Stoltenberg said.

"Together this is the most serious escalation of the conflict since the start and the aim of President Putin is to deter us from supporting Ukraine. But he will not succeed in that," he told a news conference.

He was speaking after Putin proclaimed Russia's annexation of a swathe of Ukraine at a Kremlin ceremony, promising Moscow would triumph in its "special military operation" against Kyiv even as some of his troops faced potential defeat.

Putin's proclamation of Russian rule over 15% of Ukraine - the biggest annexation in Europe since World War Two - has been firmly rejected by Western countries and even many of Russia's close allies.

Stoltenberg said Russia's move was "the largest attempted annexation of European territory by force since the Second World War," adding that an area roughly the size of Portugal had been "illegally seized by Russia at gunpoint."

"The sham referendums were engineered in Moscow and imposed on Ukraine in total violation of international law. This land grab is illegal and illegitimate.

"NATO allies do not and will not recognize any of this territory as part of Russia," he added. "We call on all states to reject Russia's blatant attempts at territorial conquest These lands are Ukraine."

He said NATO was not a party to the conflict, but it reaffirmed its "unwavering support" for Ukraine's independence, sovereignty and territorial integrity, and would continue to support the country in defending itself against Russia.