Mount Qassioun, which overlooks Damascus, is home to one of Assad's three residences and is widely used by elite forces to shell suburbs of the capital. The powerful Republican Guard and army's Fourth Division, headed by Assad's younger brother, Maher, has bases there.
A senior UN diplomat, speaking on condition of anonymity because some of this material was from private meetings, said: "It was 100 per cent clear that the regime used chemical weapons."
The diplomat cited five key details, including the scale of the attack, the quality of the sarin, the type of rockets, the warheads used and the rockets' trajectory.
A Human Rights Watch report also said the presumed flight path of the rockets cited by the UN inspectors' report led back to a Republican Guard base in Mount Qassioun.
"Connecting the dots provided by these numbers allows us to see for ourselves where the rockets were likely launched from and who was responsible," said Josh Lyons, a satellite imagery analyst for the New York-based group. But, he added, the evidence was "not conclusive".
The HRW report matched what several experts concluded after reading the UN report. The UN inspectors were not instructed to assess which side was responsible for the attack.
"While the UN stuck within its mandate, it has provided enough data to provide an overwhelming case that this had to be government-sponsored," said Anthony Cordesman, national security expert at the Center for Strategic and International Studies.
The inspectors described the rockets used to disperse the sarin as a variant of an M14 artillery rocket, with either an original or an improvised warhead, which the rebels are not known to have.
There is no conceivable way to prove the rebels could not have gotten them, Cordesman said, but he added that the modification of the rockets pointed to the regime.
The UN diplomat in New York pointed to citations in the UN report and a private briefing to the UN Security Council by chief inspector Ake Sellstrom that reveal the scale of the attack: The seven rockets examined had a total payload of about 350 litres of sarin, including sophisticated stabilising elements that match those known to be in the Syrian stockpile.
This makes it "virtually impossible" that it came from any source other than the Syrian government, the diplomat said, adding that there were likely other rockets used that the inspectors couldn't get to.
The diplomat added that the trajectory points directly at known Syrian military bases. "There isn't a shred of evidence in the other direction," he said.
Syrian legislator Issam Khalil denied the Human Rights Watch report.
"These rockets were fired by terrorists in order to draw a military act against Syria," Khalil told The Associated Press in Damascus. "We believe that a fair, transparent and objective international investigation is the only way to specify that side responsible for firing these rockets."
Russia has been Syria's main ally since the conflict began in March 2011, blocking proposed UN resolutions that would impose sanctions on Assad's regime and opposing an attempt to authorise the use of force if Syria does not abide by the agreement struck September 14 between Moscow and Washington to rid Damascus of its chemical weapons stockpile.
According to a top Russian diplomat and a Syrian official, Damascus has turned over materials to Russia that aim to show the chemical weapons attack was carried out by the rebels.
The ITAR-Tass news agency quoted Russia's Deputy Foreign Minister Sergei Ryabkov as saying that Syria told Russian officials the material it handed over shows "rebels participating in the chemical attack," but that Moscow has not yet drawn any conclusions.
Ryabkov also told pro-Kremlin broadcaster Russia Today that Russia has submitted to the UN Security Council what Moscow called credible evidence that suggests the Syrian government did not fire the chemical weapons.
"We are unhappy about this (UN) report, we think that the report was distorted, it was one-sided, the basis of information upon which it was built is insufficient," Ryabkov said.
The reports did not specify the nature of the new material turned over by Syria to Russia, which Ryabkov said would be closely analysed.
According to ITAR-Tass, Ryabkov said Russia was "inclined to treat with great seriousness the material from the Syrian side about the involvement of the rebels in the chemical attack of August 21".
UN spokesman Martin Nesirky said the UN is checking with Russia's UN Mission to find out exactly what Ryabkov said but "on the face of it, these reported remarks are an attempt to call into question the secretary-general's investigation team ... and the credibility of its thoroughly objective report".
He stressed that Secretary-General Ban Ki-moon "has the fullest confidence in the professionalism of his team and their work and findings".
The chief UN chemical weapons inspector said his team will return to Syria "within weeks" to complete the investigation it had started before the August 21 attack and other alleged uses of chemical weapons in the country.
Sellstrom told The Associated Press the team will evaluate "allegations of chemical weapons use from both sides, but perhaps mainly from the Syrian government's side".
He said he doesn't currently think there is a need for more investigations of the August 21 attacks, but said "if we receive any additional information it will be included next time we report".
The first step in getting rid of Syria's chemical weapons is for the Organisation for the Prohibition of Chemical Weapons to endorse the agreement reached by the US and Russia to put its stockpile and precursors under international control for later destruction. A senior UN diplomat said a US-Russia draft spelling out details of how this will be done is expected to be circulated to members of the OPCW's executive board later Wednesday. The board is scheduled to meet Friday to make a decision.
Britain's UN Ambassador Mark Lyall Grant said the the main purpose of a new UN resolution currently under discussion "is to make the framework agreement reached between the United States and Russia in Geneva, and the decision that will be taken by the OPCW Executive Council, legally binding in a Security Council resolution that is verifiable and enforceable".
The five permanent members of the Security Council were meeting again Wednesday to try to agree on the text.
Assad on Wednesday received a US delegation of former members of Congress and anti-war activists, including former US Attorney General Ramsey Clark.
In the contested northern city of Aleppo, a group of volunteers learned how to deal with chemical weapons attacks in a drill inside a school. Their teacher, Mohammad Zayed, a 21-year-old former chemistry student, helped them put on gas masks and protective suits.
He also described the effects of various chemical weapons and how to help people with the limited resources available.
Three gas masks and 24 protective suits were given to them after rebels gained control of a military base belonging to forces loyal to Assad. The volunteers are distributing leaflets to residents on how to react to an attack.
- AP
