U.N. experts trying to establish what killed hundreds of civilians in rebel-held suburbs of Damascus last Wednesday were finally able to cross the frontline on Monday to see survivors - despite being shot at in government-held territory. But they put off a second visit until Wednesday.
However, U.S. officials said President Barack Obama already had little doubt Assad's forces were to blame. Turkey, Syria's neighbour and part of the U.S.-led NATO military pact, called it a "crime against humanity" that demanded international reaction.
The Syrian government, which denies using gas, said it would press on with its offensive against rebels around the capital. Foreign Minister Walid al-Moualem said U.S. strikes would help al Qaeda allies but called Western leaders "delusional" if they hoped to aid the rebels to create a balance of power in Syria.
In Britain, whose forces have supported the U.S. military in a succession of wars, Cameron called for an appropriate level of retribution for using chemical weapons.
"Our forces are making contingency plans," a spokesman for Cameron told reporters. London and its allies would make a "proportionate response" to the "utterly abhorrent" attack.
Top generals from the United States and European and Middle Eastern allies met in Jordan for what could be a council of war.
GASSING "UNDENIABLE, INEXCUSABLE"
On Monday, U.S. Secretary of State John Kerry said: "President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people ... What we saw in Syria last week should shock the conscience of the world.