Syria's opposition urged the UN Security Council on Friday to act immediately, possibly even by imposing a no-fly zone, after the United States said for the first time the regime has probably used chemical weapons.
The call came as British Prime Minister David Cameron said growing evidence of chemical weapons use by President Bashar al-Assad was "extremely serious" and called for increased foreign pressure on his regime.
"It is time for the UN Security Council to act" on Syria, an official from the main opposition National Coalition told AFP on condition of anonymity because he is not authorised to speak to the media.
"This is a massive issue, and the Security Council's paralysis over Syria is no excuse," the Coalition official said.
"The UN needs to immediately investigate the use of chemical weapons in Syria. Should it find the regime used such weapons, it must act immediately, at least by imposing a no-fly zone," he added.
During the 2011 uprising that ousted long-time Libyan dictator Moamer Kadhafi, the Security Council imposed a no-fly zone to prevent civilians being hit by regime air strikes.
But the Security Council has been stalled over Syria for more than two years, with permanent members Russia and China backing Assad and vetoing several draft resolutions that would have threatened sanctions on the regime.
The Coalition has accused the regime of using chemical weapons in the northern province of Aleppo, in Homs in the centre and in rebel-held areas near Damascus.
Britain's Times newspaper published a report on Friday detailing the killing in Aleppo of a family, allegedly by chemical arms.
The family "died twitching, hallucinating and choking on white froth that poured from their noses and mouths. Their doctors believe that they were killed by nerve gas," said the report.
On Thursday, the United States said for the first time Syria had likely used chemical weapons against rebel forces, but emphasised that spy agencies were still not 100 percent sure of the assessment.
US intelligence services had been investigating reports that Assad's forces had used chemical arms -- a move President Barack Obama has said would cross a "red line".
A senior White House official said "all options are on the table" should use of the weapons be confirmed, but a defence official stressed that military intervention was not imminent and signalled spy agencies had differing opinions.
Britain's Foreign Office confirmed it also had "limited but persuasive" evidence of the use of chemical agents in the conflict, which the UN says has left more than 70,000 dead since March 2011.
Cameron said on Friday that the international response would likely be political rather than military.
"This is extremely serious. And I think what President Obama said was absolutely right, that this should form for the international community a red line for us to do more," Cameron told the BBC.
"In my view what we need to do... is shape that opposition, work with them, train them, mentor them, help them so we put the pressure on the regime and so we can bring this to an end," he said.
The European Union was cautious, reiterating a request to Damascus to allow a UN chemical weapons probe in Syria.
"We hope there will be a United Nations investigation inside Syria to hopefully shed some light on what has really happened," a spokesman for top foreign police chief Catherine Ashton said after being queried over the EU stand.
On Thursday, UN leader Ban Ki-moon renewed an "urgent call" for Syria to let inspectors into the country.
Syria asked for a UN investigation but has since refused to let a UN team waiting in the region into the country.
Assad's government, which has systematically denied it would use chemical weapons even if it had them, only wants its claims that opposition rebels used such arms to be investigated.
Ban has said the team should also look into opposition claims.
Israel was at the forefront of calls for military action.
"From the moment the international community understands that red lines have been crossed and that chemical weapons have been used, they will realise there's no other choice than to react (militarily)," said Deputy Foreign Minister Zeev Elkin.
Experts say containing the threat of chemical weapons would likely involve a presence of troops on the ground.
David Reeths, director of IHS Jane's Consulting, said "weapons of mass destruction elimination operations, as these efforts are termed, are extremely complex and would almost certainly require a significant in-country presence for an extended period of time."
Meanwhile, fresh fighting erupted on the outskirts of Damascus, as troops battled rebels in the north, south and east, backed in some areas by tanks and militia, the Syrian Observatory for Human Rights said.