"I condemn Russia's decision to extend recognition to the self-proclaimed 'Donetsk People's Republic' and 'Luhansk People's Republic'," Stoltenberg said in a statement.

"This further undermines Ukraine's sovereignty and territorial integrity, erodes efforts towards a resolution of the conflict, and violates the Minsk Agreements, to which Russia is a party," he added.

"Moscow continues to fuel the conflict in eastern Ukraine by providing financial and military support to the separatists. It is also trying to stage a pretext to invade Ukraine once again."

He urged Russia "to choose the path of diplomacy" and to withdraw its more than 150,000 troops deployed to Ukraine's borders for what many western countries expect will be an imminent attack.

Ukraine is not a NATO member country, and Stoltenberg has said NATO forces will not be sent in to defend it from Russia.

But allies, particularly the United States, have been sending weapons and other supplies to Ukraine, and have sent forces to neighbouring countries that are part of NATO.