Russia's continued support for Syrian President Bashar al-Assad is only serving to prolong the crisis there, NATO Secretary-General Jens Stoltenberg said on Monday.

Speaking to NATO's parliamentary assembly meeting in Norway, he said: "Russia should play a constructive role in the fight against (Islamic State). To support the Assad regime is not constructive. This is only prolonging the war in Syria."

Russia, Assad's main international backer, has sent its warplanes to strike anti-Assad opposition forces despite the United States and its allies urging Moscow to withdraw support from the Syrian leader and concentrate its firepower on Islamic State militants.

Tensions have further mounted with NATO accusing Russian warplanes of violating the airspace of NATO member Turkey during their sorties.

On the question of NATO troop levels in Afghanistan, Stoltenberg said NATO allies were continuing discussion of a planned withdrawal by the end of 2016.

But he said the Alliance would not leave Afghanistan entirely. "NATO is not going to leave Afghanistan, NATO will stay in Afghanistan. What we are discussing is in what way are we going to support the Afghans.

"Even if the support mission ends at the end of 2016 we will not leave Afghanistan. The main idea is that we will continue to support in one way or another," he said.