There is "compelling" evidence that Bashar Assad's regime launched a chemical weapons attack on the outskirts of Damascus last week, US Secretary of State John Kerry said as he put the death toll at more than 1,400.
At least 1,429 people were killed including 426 children, he told reporters in Washington. Mr Kerry said the American intelligence community has "high confidence" in its assessments, which are based on thousands of sources.
He added: "We know that after a decade of conflict the American people are tired of war. Believe me, I am too.
"But fatigue does not absolve us of responsibility. Just longing for peace does not necessarily bring it about and 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."
Britain's "special relationship" with America has come under intense scrutiny after Prime Minister David Cameron was forced to rule out military action following Thursday night's humiliating Commons defeat.
Mr Kerry said the US had "friends ready to respond", referring to "our oldest ally, the French", while the UK remained absent from the mix. The Secretary of State repeatedly insisted that the US "knows" what happened in the August 21 attack and has declassified some of the evidence to allow the public access to it.
He said: "We know rockets came only from regime controlled areas and went only to opposition controlled or contested neighbourhoods.
"And we know, as does the world, that just 90 minutes later all hell broke loose on the social media. With our own eyes we have seen reports from 11 separate sites in the Damascus suburbs. All of them show and report victims with breathing difficulties, people twitching with spasms, coughing, rapid heart beats, foaming at the mouth and death.
"We know it was ordinary Syrian citizens who reported all of these horrors. And, just as important, we know what the doctors and the nurses who treated them didn't report. Not a scratch, not a shrapnel wound, not a cut, not a gunshot wound."
He added: "The United States government now knows that at least 1,429 Syrians were killed in this attack, including at least 426 children."