"This land grab is illegal and illegitimate. NATO allies do not and will not recognize any of this territory as part of Russia," NATO chief Jens Stoltenberg said.

Russian President Vladimir Putin claimed the territories occupied by his troops at a grand ceremony in the Kremlin, in a move rejected by the West and UN.

Stoltenberg insisted that NATO would continue to support Ukraine "in their efforts to liberate" the Moscow-held regions as Kyiv vows to press an offensive.

The Kremlin says it now views the annexed territory as a part of Russia and is willing to use its full arsenal to defend it.

Stoltenberg said NATO had not seen any "changes in nuclear posture" of Russia after threats from Putin.

"We are vigilant, we are sharing information, and we have conveyed very clearly to Russia that there will be severe consequences if they use nuclear forces against Ukraine," Stoltenberg said.

NATO has refused to intervene militarily in the war in Ukraine for fear of unleashing a confrontation with Moscow that could spiral into nuclear war.

NATO allies have instead sent billions of dollars' worth of arms to Ukraine to help it defend itself against Moscow's attack.

In response to Putin's declaration, Ukraine announced that it was formally applying to join NATO.

Stoltenberg remained non-committal on the bid, recalling that a decision on membership required "consensus" of all 30 allies.

"Our focus now is on providing immediate support to Ukraine to help Ukraine defend itself against the Russian brutal invasion," he said.

Despite the application from Kyiv, there appear few prospects of Ukraine joining NATO in the foreseeable future while it is at conflict with its giant neighbour.

Membership of NATO would mean Ukraine was covered by the alliance's Article 5 mutual defence clause that means an attack on one member is considered an attack on all.

Stoltenberg also addressed leaks in the Nord Stream pipelines running from Russia to Europe, calling them "attacks" and backing a probe to find those responsible.

He said that NATO naval and air forces in the region showed the alliance's "readiness to protect" its critical infrastructure.