Kerry on Monday made several veiled warnings to Russia, which has propped up Assad's regime, blocked action against Syria at the U.N., and disputed evidence of the government's chemical weapons use.
"Anyone who can claim that an attack of this staggering scale can be contrived or fabricated needs to check their conscience and their own moral compass," he said.
British Prime Minister David Cameron, who like Kerry cut short his vacation because of the attack, spoke Monday with Russian President Vladimir Putin to outline the evidence of chemical weapons use by Assad's regime.
Cameron's office also said the British government would decide on Tuesday whether the timetable for the international response means it will be necessary to recall lawmakers to Parliament before their scheduled return next week. That decision could offer the clearest indication of how quickly the U.S. and allies plan to respond.
More than 100,000 people have died in clashes between forces loyal to Assad and rebels trying to oust him from power over the past two and a half years. While Obama has repeatedly called for Assad to leave power, he has resisted calls for a robust U.S. intervention, and has largely limited American assistance to humanitarian aid. The president said last year that chemical weapons use would cross a "red line" and would likely change his calculus in deciding on a U.S. response.
Last week's attack in the Damascus suburbs is a challenge to Obama's credibility. He took little action after Assad used chemical weapons on a small scale earlier this year and risks signaling to countries like Iran that his administration does not follow through on its warnings.
Syrian activists say the Aug. 21 attack killed hundreds; the group Doctors Without Borders put the death toll at 355 people.