Mr Kerry explained US intelligence showed 1429 people were killed in last week's attack, including 426 children. The total is more than four times the original estimate. The intelligence report referred to intercepted communications and evidence based on satellite information and videos.
It claimed Syrian regime personnel were at the site of the attack, a regime-controlled area, for three days beforehand, making preparations, and they had been told to put on gas masks.
As US security officials held more briefings for the US Congress, Mr Kerry made an impassioned television presentation. Labelling Bashar al Assad, the Syrian president, a thug and a murderer, he said he recognised America was war-weary, but fatigue did not absolve it of responsibility.
"History would judge us all extraordinarily harshly if we turned a blind eye to a dictator's wanton use of weapons of mass destruction against all warnings, against all understanding of common decency," he said.
He did not mention Britain's refusal to become involved in military action following a Commons vote, but he stressed the US had "friends ready to respond" and referred to "our oldest ally - the French".
Last night, Prime Minister David Cameron had a telephone conversation with Mr Obama. The president insisted the special relationship remained intact, while Mr Cameron assured him he still believed in a robust response against the Syrian government.
Reports from Damascus suggested the regime was moving prison inmates to areas it believed could be targets for US-led military strikes. It is thought allied attacks will take place after the UN inspectors leave Syria today and before Mr Obama leaves for the G20 meeting in Russia on Tuesday.