Syrian government forces killed 1,429 people in a chemical weapons attack in Damascus last week, said US Secretary of State John Kerry.
John Kerry said the dead included 426 children, and described the attack as an “inconceivable horror”.
The US is pushing for intervention to stop the Syrian government from using chemical weapons.
The government of President Bashar al-Assad has denied carrying out the attack and blames rebel forces.
Medical charities had previously said the attack in eastern Damascus on August 21 had killed 355.
But John Kerry said the US now had the facts, which showed 1,429 people had been killed.
He said Assad regime forces had prepared for the attack three days earlier.
“We know rockets came only from regime-controlled areas and landed only in opposition-held areas,” he said.
“All of these things we know, the American intelligence community has high confidence.”
John Kerry said the government would consult Congress and the American people over the next step.