NATO Secretary-General Anders Fogh Rasmussen has said that a variety of sources pointed to President Bashar al-Assad's forces being responsible for the use of chemical weapons in Syria.

Speaking after a meeting of NATO ambassadors in Brussels, Rasmussen said any use of such weapons was "unacceptable and cannot go unanswered", although he did not suggest any response.

"This is a clear breach of long-standing international norms and practice... those responsible must be held accountable," he said in a statement.

Rasmussen said the military alliance would keep the situation in Syria under "close review."

His comments came as the United States and its allies pressed their case for likely military action against Assad's government, despite stern warnings against intervention from Damascus' key allies Russia and Iran.

Britain plans to submit a draft resolution to the UN Security Council on Wednesday which will condemn suspected chemical attacks in Syria and demand "all necessary measures" to protect civilians.

Britain's national security council met on Wednesday and unanimously backed action against Syria, and Prime Minister David Cameron has recalled parliament from a summer recess for a debate on the crisis.

'International law clear'

Lakhdar Brahimi, the UN and Arab League envoy for Syria, said that international military action could be carried out only after it has been approved by the Security Council.

"I think international law is clear on this. International law says that military action must be taken after a decision by the Security Council," he told reporters in Geneva.

Syria has denied that it carried out the attack, and challenged world powers to provide evidence that it had done so. The Syrian ambassador to the United Nations, Bashar al-Jaafari, on Wednesday blamed the rebels for the attacks and said they were carried out with the help of Turkey, Saudi Arabia and Qatar.

Meanwhile, UN chemical weapons experts investigating the attack that killed hundreds of civilians in rebel-held suburbs of Damascus finished a second trip to Ghouta to take samples.

The inspectors' first such visit on Monday was briefly suspended after the UN's convoy came under sniper fire from unidentified gunmen, though they did visit two field hospitals to collect evidence.

UN Secretary-General Ban Ki-moon said it would take the inspectors four days to finish gathering evidence, and they would then need time to analyse their findings.

Al Jazeera’s James Bays, reporting from the UN headquarters in New York, said the world body keeps saying that the inspectors need time to find evidence on the use of chemical weapons.

“This [the delay in the work of the inspectors] presents a little of a problem for US, UK and France as they consider possibly taking military action without UN authorisation," he said.

Meanwhile, Iran's supreme leader, Ayatollah Ali Khamenei, said on Wednesday that US intervention in Syria would be "a disaster for the region", the ISNA state news agency reported.

"The intervention of supra-regional and foreign powers in one country will have no result other than lighting a fire and increase the hatred people have for them," the gency quoted Khamenei as saying.

"This lighting of a fire is like a spark in a gunpowder magazine whose dimensions and consequences are unknown."

Turkey has put its armed forces on alert, foreign minister Ahmet Davutoglu told reporters on Wednesday.

On Tuesday, US Vice-President Joe Biden became the highest ranking US official to accuse Syria's government of having carried out the attack, asserting that there was "no doubt" that the government was "responsible for this heinous" act.

Also on Tuesday, Chuck Hagel, the US Defence Secretary, repeated previous statements that US forces were now positioned to strike Assad's forces should the decision to take military action be made.