In May, a U.N. official said there were strong suspicions that Syrian rebel forces had used sarin gas. But the findings were not conclusive, the U.N. Independent International Commission of Inquiry for Syria said at the time, and the opposition Syrian Coalition condemned any use of chemical weapons. The U.S. State Department said at the time it had no evidence suggesting rebels had used chemical weapons.
The Russian Foreign Ministry said Wednesday the results of an investigation into a March attack in Aleppo, apparently using chemical weapons, found that the charge used was homemade and similar to projectiles produced by the group Bashaar al-Nasr, part of the opposition Syrian Islamic Liberation Front. Sarin was discovered in samples from the scene, the foreign ministry said.
Bashaar al-Nasr has slammed Syria for the recent chemical weapons attack in Damascus, and vowed revenge.
U.S.: Syria prepared
"In the three days prior to the attack, we collected streams of human, signals and geospatial intelligence that reveal regime activities that we assess were associated with preparations for a chemical weapons attack," the U.S. report says.
"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