Momentum appears to be building for Western military action against Syria, with the US and France saying they are in position for a strike, while the government in Damascus vowed to use all possible measures to repel it.



The prospect of a dramatic US-led intervention into Syria's civil war stemmed from the West's assertion - still not endorsed by UN inspectors - that President Bashar Assad's government was responsible for an alleged chemical attack on civilians outside Damascus on August 21 that the group Doctors Without Borders says killed 355 people. Assad denies the claim.



The Arab League also threw its weight behind calls for punitive action, blaming the Syrian government for the attack and calling for those responsible to be brought to justice.



British Prime Minister David Cameron recalled Parliament to hold an emergency vote on Thursday (local time) on his country's response. It is unlikely that any international military action would begin before then.



US Defence Secretary Chuck Hagel said US military forces stand ready to strike Syria at once if President Barack Obama gives the order, and French President Francois Hollande said France was "ready to punish those who took the heinous decision to gas innocents."



Obama is weighing a response focused narrowly on punishing Assad for violating international agreements that ban the use of chemical weapons. Officials said the goal was not to drive Assad from power or impact the broader trajectory of Syria's bloody civil war, now in its third year.



Russian Foreign Minister Sergey Lavrov said on Monday the West should be under no illusion that bombing Syrian military targets would help end the violence in Syria, an ally of Moscow, and he pointed to the volatile situations in Iraq and Libya that he said resulted from foreign military intervention.



Syrian Foreign Minister Walid al-Moallem said his country would use "all means available" to defend itself.



"We have the means to defend ourselves and we will surprise everyone," he said.



At a news conference in Damascus, al-Moallem challenged Washington to present proof to back up its accusations and he also likened the allegations to false American charges in 2003 that Iraq possessed weapons of mass destruction before the US-led invasion of that country.



"They have a history of lies - Iraq," he said.



Vice President Joe Biden said there was no question that Assad was responsible for the attack - the highest-ranking US official to say so - and the White House dismissed as "fanciful" the notion that anyone other than Assad could be to blame.



"Suggestions that there's any doubt about who's responsible for this are as preposterous as a suggestion that the attack did not occur," spokesman Jay Carney said.



A US official said some of the evidence includes signals intelligence - information gathered from intercepted communications. The US assessment is also based on the number of reported victims, the symptoms of those injured or killed, and witness accounts. The officials insisted on anonymity because they were not authorised to publicly discuss the internal deliberations.



The United Nations said its team of chemical weapons experts in Syria had delayed a second trip to investigate the alleged attack by one day for security reasons. On Monday, the team came under sniper fire.



If Obama decides to order an attack against Syria, it would most likely involve sea-launched cruise missile attacks on Syrian military and communications targets.



Hagel said the US Navy had four destroyers in the eastern Mediterranean Sea positioned within range of targets inside Syria. US warplanes were also in the region, he told BBC television during a visit to the southeast Asian nation of Brunei.



In Cyprus, Defence Minister Fotis Fotiou said naval traffic in the eastern Mediterranean was very heavy with vessels from "all the major powers." He also said Cypriot authorities were planning to deal with a possible exodus of foreign nationals from Syria.



US military intervention in Syria was running into fierce opposition from some members of Congress. A growing chorus of Republican and Democratic lawmakers demanded that Obama seek congressional authorisation for any strikes against the Assad regime.



Charles Heyman, a former British officer who edits The Armed Forces of the UK, said the lack of a UN Security Council resolution authorising the use of force against the Syrian government greatly complicates matters for the West. He said that may make it difficult for Cameron to win parliamentary backing.



"It's clear the governments want some form of military operation, but if the Security Council doesn't recommend it, then the consensus is that it's plainly illegal under international law," Heyman said. "The only legal way to go to war is in self-defence and that claim is difficult to make."



Russia, a permanent member of the Security Council, has steadfastly opposed any international action against Syria.



Italian Foreign Minister Emma Bonino said her country would not back any military action against Syria unless it was authorised by the Security Council - even though it considers a chemical attack to be a war crime.



German Foreign Minister Guido Westerwelle said on Monday that if the Syrian government were proven to have been behind the gas attack, then Germany would support "consequences." But with less than four weeks until national elections, it is unlikely Germany would commit any forces.



Centre-left opposition parties have rejected military intervention without UN proof that the Syrian government was behind the attack. And a senior member of Chancellor Angela Merkel's party said the German military was already at "the breaking point" due to commitments in Afghanistan and elsewhere.



Support for some sort of international military response is likely to grow if it is confirmed that Assad's regime was responsible.



The UN confirmed its chemical weapons team's mission faced a one-day delay on Tuesday to improve preparedness and safety after unidentified snipers opened fire on the team's convoy Monday.



In Geneva, UN spokeswoman Alessandra Vellucci said the UN inspection team might need longer than the planned 14 days to complete its work. She said its goal is to determine what chemical weapons might have been used in the August 21 attack.



The Obama administration is making a legal argument for undertaking a military response to the use of chemical weapons against civilians in Syria, but said any action against the Syrian regime is not intended to depose Assad.