A boy holds a poster of Syria's President Bashar al-Assad during a demonstration against potential strikes on the Syrian government, in Sanaa August 30, 2013.[China Daily/Agencies]

A Syrian chemical weapons attack killed 1,429 Syrian civilians, including 426 children, an unclassified report by US intelligence agencies concluded on Friday.

President Barack Obama is using the report to make the case for retaliation against the Syrian government.

The report, based partly on intercepted communications, said US authorities have a high degree of confidence that the Syrian government of Bashar al-Assad was responsible, which it said is the strongest position the US intelligence community can take short of confirmation.

A central piece of intelligence included a communication that was intercepted from "a senior official intimately familiar with the attack," who confirmed that chemical weapons had been used by the government on Aug. 21 and was "concerned with the U.N. inspectors obtaining evidence" about it.

The report said a nerve agent was used in the attack, which took place in the Damascus suburbs and was aimed at ridding the area of those trying to topple the Assad government.

The death toll given by the report was the first precise number and far larger than previously estimated. A senior administration official who briefed reporters said the number could rise.

The report said the conclusion was based on human, signals and satellite intelligence as well as a significant body of public material, such as amateur videos.

The report said three hospitals in the Damascus area received some 3,600 patients showing symptoms consistent with nerve agents in less than three hours on the morning of Aug. 21.

Rejecting claims by the Syrian government that the Syrian opposition conducted the attack, the report said the rebels had no capability to fabricate all of the videos and the physical symptoms verified by medical personnel.