The ambassadors of the 30-nation alliance were to hold an emergency meeting early Thursday to discuss the Russian attack, with Stoltenberg expected to give a press conference afterwards, a NATO official said separately.

"I strongly condemn Russia's reckless and unprovoked attack on Ukraine, which puts at risk countless civilian lives," Stoltenberg said in a statement.

"Once again, despite our repeated warnings and tireless efforts to engage in diplomacy, Russia has chosen the path of aggression against a sovereign and independent country," Stoltenberg said in a statement.

"This is a grave breach of international law, and a serious threat to Euro-Atlantic security. I call on Russia to cease its military action immediately and respect Ukraine's sovereignty and territorial integrity," he said.

Stoltenberg added: "We stand with the people of Ukraine at this terrible time. NATO will do all it takes to protect and defend all allies."

The statement came after Russian President Vladimir Putin announced a military operation in Ukraine and air and missile strikes happened in multiple locations across the country.

Ukraine is not a member of the 30-nation Western military alliance but its attempt to join has angered the Kremlin leader.

Putin has demanded guarantees from NATO and the US that Ukraine would never be granted membership.

Stoltenberg has previously said NATO has no plans to send troops to fight in Ukraine if Russia attacked and key power Washington has ruled out deploying its forces to defend the country.