The former US presidential candidate said proof would be made public in the coming days that the Government carried out the attacks which reportedly claimed the lives of 1300 of its own people. He said doubters needed "to check their conscience and their own moral compass".
Mr Kerry said the credibility of Syrian President Bashar al Assad was further undermined after a convoy carrying a team of United Nations inspectors investigating the alleged use of chemical weapons came under sniper fire.
He would not elaborate on what Barack Obama's overall response might be, adding that the President "believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people".
The regime was undoubtedly responsible for a "moral obscenity" that "should shock the conscience of the world", he said.
Russia warned against using military action in Syria without a Security Council mandate.
Prime Minister David Cameron will today decide whether to recall Parliament under mounting pressure from some MPs who fear he may sanction quick air strikes.
SNP MP Angus Robertson said: "The UK Parliament must be updated on developments and consulted on a potential international military intervention."
Labour MP Thomas Docherty said it would be "absolutely unforgiveable" for the Coalition to consider military options without recalling Parliament.
Foreign Secretary William Hague said diplomacy had failed and military action "may be the choice." He added: "This is why we have called for a strong response." Russian President Vladimir Putin told Mr Cameron by phone there was still no evidence of a chemical attack nor, if one had taken place, who was responsible.