"This land grab is illegal and illegitimate. NATO allies do not and will not recognise any of these territories as part of Russia," NATO Secretary Stoltenberg said.
On Friday, White House National Security Adviser Jake Sullivan and North Atlantic Treaty Organization (NATO) Secretary Jens Stoltenberg expressed their "firm commitment" to "the sovereignty and territorial integrity of Ukraine."
Their statements came after the ceremony by which President Vladimir Putin signed the accession treaties of the Donetsk, Lugansk, Kherson, and Zaporizhia regions to Russia.
During a phone conversation, Sullivan and Stoltenberg shared their concern about Russia's "illegitimate" attempts to annex territory from Ukraine through referendums.
"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 recognise any of these territories as part of Russia," Stoltenberg.
The phone call coincides with the Ukrainian request to join NATO, which President Volodymyr Zelensky made in response to the admission of the four regions to the Russian federation.
Sullivan and Stoltenberg also spoke of the "alleged sabotage" of the Nord Stream pipelines in international waters and the need to protect "crucial infrastructure".
On Friday, the U.S. President Joe Biden accused Russia of violating international law due to the "fraudulent attempt" to annex Ukrainian territories.
"We will continue to help Ukraine in its efforts to regain control over its territories," he said after the accession ceremony held in the Kremlin.