The Arab League and United Nations special envoy to Syria, Lakhdar Brahimi, has said that some kind of chemical substance was used in the attack on Ghouta but a reprisal strike by the West against Syrian government facilities must have UN Security Council approval.
As British prime minister David Cameron announced that a draft resolution authorising "necessary measures" in Syria would be presented to the council within hours, Brahimi said that under international law any military action must only be taken after agreement in the 15-nation Security Council.
He added that Obama's administration was "not known to be trigger-happy".
Earlier, UN secretary-general Ban Ki-moon said inspectors should be given time to determine whether Assad's forces used toxic gas against civilians.
However, the US State Department maintained that it was already "crystal clear" that Assad was responsible for the attack, in which at least 300 people died.
"Chemical weapons have been used," said vice-president Joe Biden.
Any resolution put forward at a meeting of the five permanent members of the Security Council in New York is likely to be vetoed by Russia and China, who are allies of the Assad regime.
However, Germany has called on Russia to back the resolution.
"We welcome the British initiative to get the UN Security Council to again consider the use of chemical weapons in Syria. We urge all members of the Security Council, in particular Russia, to seize this opportunity and contribute to a common stance by the global community against the use of chemical weapons of mass destruction in Syria," Foreign Minister Guido Westerwelle said.