NATO's secretary-general has rejected Moscow's claim that its military incursion into alliance airspace over Turkey wasn't intentional or important, saying there were two separate incidents and "the violation lasted for a long time."

Jens Stoltenberg, the NATO chief, told a news conference in Brussels on Tuesday the reported incidents were "very serious."

Stoltenberg added, "It doesn't look like an accident, and we've seen two of them over the weekend."

On Monday, NATO ambassadors met in special session and condemned what they termed Russia's "irresponsible behavior."

The ambassadors also called on Russia to cease such practices.