"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable.
"And despite the excuses and equivocations that some have manufactured, it is undeniable."
How such an intervention, likely to be limited to some form of air strike, would affect the course of Syria's civil war is far from clear. Obama, Cameron and French President Francois Hollande face tough questions on how far they want to use force to achieve a long-stated common goal of forcing Assad from power.
Turmoil in Egypt, whose 2011 uprising inspired Syrians to rebel, has underlined the unpredictability of revolutions. And the presence of Islamist militants, including allies of al Qaeda in the Syrian rebel ranks, has given Western leaders pause. They have held back so far from helping Assad's opponents to victory.
Russia, a major arms supplier to Assad, has said rebels may have released the gas and warned against attacking Syria. Deputy Foreign Minister Gennady Gatilov criticised Washington for cancelling bilateral talks on Syria that were set for Wednesday.
"Working out the political parameters for a resolution in Syria would be exceptionally useful now, when the threat of force hangs over this country," Gatilov wrote on Twitter.