NATO Secretary-General Jens Stoltenberg rejected Russia’s "illegal and illegitimate” annexation of four parts of Ukraine.
Stoltenberg said NATO allies support Ukraine's right to choose its own path.
His comments came shortly after Ukraine announced that it would apply for accelerated membership in the bloc.
"This is the largest attempted annexation of European territory by force since World War II," Stoltenberg said in a press conference on Friday.
"NATO Allies do not, and will not, recognize any of this territory as part of Russia," he said, calling on all states to reject the annexation.
The top NATO official said the move "represents the most serious escalation since the start of the war" and pledged further support to Ukraine.
