US Secretary of State John Kerry has said Syrian government forces killed 1,429 people in a chemical weapons attack near Damascus last week.
Mr Kerry said the dead included 426 children, and described the attack as an “inconceivable horror”.
He said that any response would not involve the US in a protracted conflict like Iraq or Afghanistan.
The government of President Bashar al-Assad has denied carrying out last week’s attack and blames rebel forces.
UN chemical weapons inspectors are investigating the alleged poison-gas attacks and will present their evidence to the UN after they leave Damascus on Saturday.
But Mr Kerry said the US already had the facts, and nothing that the UN weapons inspectors found could tell the world anything new.
The US government earlier published an assessment of its intelligence, saying this information was backed by accounts from medical personnel, witnesses and journalists, videos and thousands of social media reports.
He said the evidence showed 1,429 people had been killed and that 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.”
Mr Kerry called Mr Assad “a thug and a murderer” but said any response by the US would be carefully measured to avoid open-ended commitments.
The UN Security Council is unlikely to approve any military intervention because permanent member Russia is a close ally of the Syrian government, and has vetoed two previous draft resolutions.
The US was also dealt a blow on Thursday when the UK parliament rejected a motion supporting the principle of military intervention.
The vote rules the UK out of any potential alliance.
However, US officials said they would continue to push for a coalition, and France said it would support the US.
The use of chemical weapons is banned under several treaties, and is also considered illegal under customary international humanitarian law.
The Syrian army is known to have stockpiles of chemical agents including sarin gas.
Earlier accounts of the attack in Damascus quoted officials from medical charity Medicins Sans Frontieres as saying 355 people had been killed.
Opposition sources later claimed more than 1,000 people had died.
The UN inspectors have collected various samples that will now be examined in laboratories across the world.
The UN team is not mandated to apportion blame for the attacks.
More than 100,000 people are estimated to have died since the conflict erupted in Syria in March 2011, and the conflict has produced at least 1.7 million refugees.
BBC © 2013