He said NATO allies "will meet to address the consequences of Russia's aggressive actions".

"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 explosions were heard in multiple locations across the country.

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

Putin has demanded guarantees from NATO and the US that Ukraine would never be granted membership as he has massed huge forces along the border with Russia's pro-Western neighbour.

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.