NATO head Anders Fogh Rasmussen said he is "personally convinced" that Syrian President Bashar al-Assad used chemical weapons near Damascus last month.
"Personally, I am not only convinced that a chemical attack has taken place ... I am also convinced that the Syrian regime is responsible," Rasmussen said when asked if he had seen intelligence to prove Assad was behind the suspected attack which left hundreds dead.
"In general, we have concrete information," he said.
Rasmussen stressed repeatedly that the international community could not allow the use of chemical weapons go by unchallenged but he was guarded about any military response.
"The aim of the response should be to send a very clear message ... that you cannot do that without a very firm international reaction," he told a regular news conference.
Any military action, however, "would be very short, sharp, tailored," Rasmussen said, adding: "I don't see a long-term, sustainable military solution in Syria ... we need a political process."
It would be up to NATO's 28 members, led by the United States, to decide what specific military response they might make, he said.
"I don't envisage any further NATO role" beyond providing a forum for consultations and backing up member Turkey which shares a tense border with Syria, he added.
Earlier this year, NATO deployed Patriot anti-missile batteries supplied by the United States, Germany and the Netherlands to help defend Turkey after border villages were hit by fire from inside Syria.
The Patriot missiles have a deterrent effect, Rasmussen said when asked about a possible change in NATO's stance if Syria were to retaliate against Turkey for a US strike.
"An attack on one ally is an attack on all," he said, recalling NATO's core rationale and mission.
"I can assure you we have all plans in place for an effective defence of Turkey ... I don't see any need for extra contingency planning."
Western powers face considerable public reluctance to get involved in another Middle East conflict and US President Barack Obama is trying to drum up Congressional backing for his plans to punish Assad.
Britain has backed out of any military commitment after a parliamentary vote last week blocked efforts by Prime Minister David Cameron to join US forces in military action, leaving France the only major European country still committed to a strike.
Earlier, Syrian Deputy Foreign Minister Faisal Mekdad warned that any US military action against Syria would amount to "support for al-Qaeda and its affiliates".
"Any attack against Syria is support for al-Qaeda and its affiliates, whether Jabat al-Nusra or the State of Islam in Syria and Iraq," Mr Mekdad told the BBC.
Mr Mekdad also warned that possible US intervention would deepen "hatred for the Americans" and destabilise the whole Middle East.
He said that Mr Obama's surprise decision to seek congressional approval for strikes showed that he had not thought through all the "consequences".
Meanwhile, Syria has reportedly asked the United Nations to try to "prevent any aggression" against it, state news agency SANA said Monday, as the United States weighs military action against Damascus.
"The Syrian government calls on the UN secretary general to assume his responsibilities... and to make efforts to prevent any aggression against Syria," the agency said, quoting a letter from Syria's UN representative Bashar al-Jaafari.
The letter also urged the United Nations to help seek a "peaceful political solution to the crisis" in Syria, where more than 110,000 people have been killed in violence since an uprising against the regime began in March 2011.
US President Barack Obama is seeking approval from Congress for military action against Syria in response to an alleged chemical weapons attack on Damascus suburbs that killed hundreds of people.
