The Syrian regime spent days carefully preparing to launch a chemical weapons attack, US Secretary of State John Kerry has disclosed.

Based on intelligence, Mr Kerry said Syrian regime personnel were at the site of the attack for three days beforehand, making preparations.

He said regime elements were told to prepare by putting on gas masks.

The US also knows where the rockets were launched from. Mr Kerry said they came from regime-controlled areas.

He said a senior regime official confirmed that the weapons were used and was afraid it would be discovered.

The US is releasing a public report on intelligence gathered about last week's deadly attack. President Barack Obama is preparing for a possible military strike in response.

The US chemical weapons assessment said Syrian President Bashar Assad's government used an unidentified nerve agent in the attack. The report cites human and satellite intelligence that it said backs up publicly available videos and other evidence.

The unclassified report said at least 426 children died.

The report said the "high confidence" assessment is the strongest position that US intelligence agencies can take short of confirmation.

It dismisses the Assad government's contention that rebels were responsible.

The U. said additional intelligence remains classified but is being provided to allies and Congress.