President Barack Obama is preparing for a possible military strike in response.
Secretary of State John Kerry says the U.S. knows based on intelligence that the Syrian regime carefully prepared for days to launch a chemical weapons attack.
The Obama administration says it has "high confidence" that Syria's government carried out the chemical weapons attack last week outside Damascus, the capital — killing 1,429 people. President Barack Obama is preparing for a possible military strike in response.
The U.S. chemical weapons assessment says Syrian President Bashar Assad's government used an unidentified nerve agent in the attack. The report cites human and satellite intelligence that it says backs up publicly available videos and other evidence.
The unclassified report says at least 426 children died.
Kerry says Syrian regime personnel were at the site of the attack for three days before, making preparations.
He says regime elements were told to prepare by putting on gas masks.
Kerry says the U.S. also knows from where the rockets were launched. He says the rockets came from regime-controlled areas.
Kerry also says a senior regime official confirmed that the weapons were used and was afraid it would be discovered.
The report says the "high confidence" assessment is the strongest position that U.S. intelligence agencies can take short of confirmation.
It dismisses the Assad government's contention that rebels were responsible.
The U.S. says additional intelligence remains classified but is being provided to allies and Congress.