John Kerry said he had "high confidence" in the
US Secretary of State John Kerry has said Syrian government forces killed 1,429 people in a chemical weapons attack in Damascus last week.
Mr Kerry said the dead included 426 children, and described the attack as an "inconceivable horror".
He did not mention military strikes, and said the government would consult Congress leaders over the next step.
The government of President Bashar al-Assad has denied carrying out the attack and blames rebel forces.
UN chemical weapons inspectors are investigating the evidence of the chemical attacks.
But Mr 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."
The US is pushing for intervention to stop the Syrian government from using chemical weapons.