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."