US Secretary of State John Kerry says he and Arab League foreign ministers have agreed that the Syrian president’s alleged use of chemical weapons had crossed a “global red line“.
Mr Kerry, speaking in Paris, is in Europe to muster support for action against President Bashar al-Assad.
“Assad’s deplorable use of chemical weapons crosses an international, global red line,” he said.
The Arab countries are divided on the question of military strikes on Syria.
The BBC’s Hugh Schofield reports from Paris that some like Saudi Arabia and Qatar are in favour while others like Syria’s neighbours Jordan and Lebanon are far more cautious, worried about the conflict spreading across their borders.
The US accuses Mr Assad’s forces of killing 1,429 people in a sarin gas attack on 21 August.
Mr Assad’s government blames the attack on rebels fighting to overthrow him in the country’s two-and-a-half-year civil war, which has claimed some 100,000 lives, according to UN estimates.
There are reports that rebel forces have taken control of the historic Christian town of Maalula, north of Damascus.
