The head of the Arab League has said military intervention in Syria is not an option - a further blow to the United States' efforts to act over a chemical weapons attack in Damascus last month.

Following emergency meetings in Cairo on Monday, secretary-general, Nabil Elaraby, said the League held the Syrian regime of Bashar al-Assad responsible for the August 21 attack, but a "military option is out of the question".

He said that the United Nations inspectors who had investigated the attack site "do not have the powers to say who committed this... so, all the inspectors will say is that chemical weapons have been used".

His comments came a day after the League stopped short of calling for military intervention in Syria, instead asking the UN and the international community to take "deterrent" measures under international law.

Elaraby added that he considered that only the UN, "as the official representative of the international community" could "take action to stop those who committed this crime".

Russia, a permanent member of the UN Security Council and an ally of Assad, has said there is no evidence that the chemical attack was launched by the Assad regime.

NATO statement

Meanwhile, the secretary general of Nato, Anders Fogh Rasmussen, said the international community could not ignore the attack.

"We believe that these unspeakable actions which claimed the lives of hundreds of men, women and children cannot be ignored," Rasmussen told a news conference.

"I think there is an agreement that we need a firm international response in order to avoid that chemical attacks take place in the future. It would send, I would say, a dangerous signal to dictators all over the world if we stand idly by and don't react," he added.

The Syrian regime, meanwhile, has asked the UN to prevent "any aggression" against Syria.

The US plans for military action against Syria will be put to a vote in the Congress, which ends its summer recess on September 9, giving Assad time to prepare for any assault and rally international support against the use of force.

Meanwhile, Bashar Jaafari, Syria's envoy to the UN, said in a letter that the UN secretary-general, Ban Ki moon, should "shoulder his responsibilities for preventing any aggression on Syria and pushing forward reaching a political solution to the crisis in Syria".

He called on the Security Council to "maintain its role as a safety valve to prevent the absurd use of force out of the frame of international legitimacy".

Jaafari said the US should "play its role, as a peace sponsor and as a partner to Russia in the preparation for the international conference on Syria and not as a state that uses force against whoever opposes its policies".

Syria denies using chemical weapons and accuses rebel groups, who have been fighting for more than two years to topple Assad, of using the banned weapons.

At least 100,000 people have been killed in the Syrian conflict, which started in March 2011 with protests against four decades of Assad family rule.