"Syrian chemical weapons personnel were operating in the Damascus suburb of 'Adra from Sunday, August 18 until early in the morning on Wednesday, August 21 near an area that the regime uses to mix chemical weapons, including sarin. On August 21, a Syrian regime element prepared for a chemical weapons attack in the Damascus area, including through the utilization of gas masks. Our intelligence sources in the Damascus area did not detect any indications in the days prior to the attack that opposition affiliates were planning to use chemical weapons."
"We have a body of information, including past Syrian practice, that leads us to conclude that regime officials were witting of and directed the attack on August 21," the U.S. report says. "We intercepted communications involving a senior official intimately familiar with the offensive who confirmed that chemical weapons were used by the regime on August 21 and was concerned with the U.N. inspectors obtaining evidence."
Intelligence shows Syrian chemical weapons personnel were told to cease operations in the afternoon of August 21, and that the regime then "intensified the artillery barrage targeting many of the neighborhoods where chemical attacks occurred," the report says.
Analyst: 'No way in hell' U.S. can back up death toll
The U.S. report says a preliminary assessment "determined that 1,429 people were killed in the chemical weapons attack, including at least 426 children."
The assessment "will certainly evolve as we obtain more information," it adds.
"Secretary Kerry seems to have been sandbagged into using an absurdly over-precise number," said Anthony Cordesman, former director of intelligence assessment at the U.S. Defense Department.
Now with the Center for Strategic and International Studies, he writes on the CSIS website, "Put simply, there is no way in hell the U.S. intelligence community could credibly have made an estimate this exact."
It's unclear whether "these figures really had an intelligence source," Cordesman said . "Some sources indicate they may have actually come from a Syrian source called the Local Coordination Committees (LCC)" -- a Syrian opposition group.
A U.S. official told CNN the number is not based on opposition figures. The methodology used to come up with the toll remains classified.
Rebel leaders have given similar estimates for the death toll, saying more than 1,300 people were killed.
Britain's Joint Intelligence Organization, meanwhile, says at least 350 people were killed. It does not say how the figure was determined.
A French government report notes that body counts by several sources, including Doctors Without Borders, estimated at least 355 deaths. "Other technical counts, using different sources, estimate the toll to be around 1,500 deaths," the report says.
Britain, France, Germany weigh in
A letter from Jon Day, chairman of Britain's Joint Intelligence Committee, to British Prime Minister David Cameron, also rejects suggestions that the opposition may have been behind the attack.
"We have tested this assertion using a wide range of intelligence and open sources, and invited (the government) and outside experts to help us establish whether such a thing is possible," Day wrote.
No "credible intelligence" suggests the opposition has chemical weapons, he writes, adding that "there are no plausible alternative scenarios to regime responsibility."
"We also have a limited but growing body of intelligence which supports the judgment that the regime was responsible for the attacks and that they were conducted to help clear the opposition from strategic parts of Damascus. Some of this intelligence is highly sensitive but you have had access to it all."
France gives a similar argument.
"The attack of August 21st could only have been ordered and carried out by the regime," its declassified intelligence report says.
Germany's foreign intelligence agency, the BND, has the same assessment.
"In a secret briefing to select lawmakers on Monday, BND head Gerhard Schindler said that while there is still no incontestable proof, analysis of the evidence at hand has led his intelligence service to believe that Assad's regime is to blame," Der Spiegel reports.
U.N. probe: Limited scope, no clear deadline
The United Nations is pushing all nations to hold off on any action until results of its own examination are in. It's unclear how soon that may be.