MOSCOW: Syrian President Bashar al-Assad is likely to fall and NATO must plan in advance to protect against the threat of his chemical arsenal falling into the wrong hands, a senior official of the Western military alliance said on Friday.

Fighting between the rebels and government forces has begun to rattle the heart of Assad's power in Damascus in a conflict that has killed at least 40,000 people since the state began to crack down on street protests in March 2011.

Western powers have repeatedly warned of consequences should Damascus use chemical weapons against the insurgents in an attempt to cling to power and has expressed concern that the stocks may fall into the hands of Islamic extremists.

"My concern is not that the Syrian armed forces will use them, but if we assume that the Assad regime in one way or another will disappear, who will control the chemical weapons?" said General Knud Bartels, head of NATO's Military Committee.

"And I do not need to tell you that there is not a big amount required to spread disaster in the metro of New York, London, Paris or Moscow," he said. "And therefore we have to do some ... thinking as to how we address this in due time."

The 60-year-old Danish general, speaking at a Russian military academy on his first visit to Moscow, said he did not know how long the civil war in Syria would continue, but said developments on the ground suggested Assad would fall.

"You may say I am maybe assuming that Assad will disappear. I tend to believe that this is indeed the case," he said.

On Thursday, NATO head Anders Fogh Rasmussen said he thought Assad's government was nearing collapse and the new leader of Syria's opposition told Reuters the people of Syria no longer needed international forces to protect them.

Answering a question about strategies in the conflict, Bartels said NATO's greatest concern was over the security of Turkey, the only alliance member that borders Syria.

NATO agreed last week to send Patriot missiles to Turkey to protect it against the threat of missiles fired from Syria.

Moscow voiced criticism of the deployment, saying it was further undermining stability in the region and accusing NATO of moving towards involvement in the conflict.

Bartels repeated NATO's assurances that the deployment would be purely defensive and was not aimed at creating a no-fly zone over part of Syria.

"There is no intention, hidden or public, directly or indirectly, to implement a no-fly zone in the airspace of Syria. There is no operational planning going on on any kind of intervention in Syria."