Putin signed documents on Friday to incorporate Ukraine's Donetsk, Luhansk, Zaporizhzhia and Kherson regions into Russia at a televised ceremony in the Kremlin.
NATO Secretary-General Jens Stoltenberg said the move was part of "the most serious escalation" of the war in Ukraine since it began in late February.
Stoltenberg told reporters: "We have the combination of the mobilisation in Russia, combined with the reckless, dangerous nuclear rhetoric, and then today's illegal annexation, or attempt to annex parts of Ukraine."
He added: "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."
Stoltenberg also said that 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 Reuters news agency reported.
"The sham referendums were engineered in Moscow and imposed on Ukraine in total violation of international law," Stoltenberg said, as quoted by Reuters.
"This land grab is illegal and illegitimate," he declared, adding that "NATO allies do not and will not recognise any of this territory as part of Russia."
"We call on all states to reject Russia's blatant attempts at territorial conquest," Stoltenberg also said. "These lands are Ukraine."
He reaffirmed NATO's "unwavering support for Ukraine’s independence, sovereignty and territorial integrity," and said that the Western alliance would continue to support the country in defending itself against Russia.
"We remain resolute in providing support to Ukraine as it continues to defend itself against Russia’s aggression," he told reporters. "For as long as it takes."
Source: IAR, PAP, Reuters, nato.int