U.N. inspectors say there is "clear and convincing evidence" that chemical weapons were used on a relatively large scale in an attack last month in Syria that killed hundreds of people.
A report from the inspectors says "the environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used ... in the Ghouta area of Damascus" on Aug. 21.
"The conclusion is that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic ... against civilians including children on a relatively large scale," the inspectors said on the first page of their report to Secretary-General Ban Ki-Moon.
Ban is scheduled to present the report to the U.N. Security Council on Monday. The Associated Press saw the first page of the report.
That report is expected to add momentum to a deal to eradicate Syria's chemical weapons program.
Pinheiro also said the panel believes Syrian President Bashar Assad's government has been responsible for war crimes and crimes against humanity, while rebel groups have perpetrated war crimes but not crimes against humanity "because there is not a clear chain of command."
U.S. Secretary of State John Kerry ended a weeklong diplomacy tour in Paris on Monday. Just a week ago he was here lobbying for global support for military strikes against Assad, but after a breakthrough with Russia, Kerry's latest visit was intended to secure support from allies for the deal to secure and then eradicate Syria's chemical weapons arsenal.
Kerry and his French and British counterparts laid out a two-pronged approach in Syria, calling for enforceable U.N. benchmarks for eradicating the chemical weapons program and an international conference bolstering the moderate opposition.
France and the U.S. insisted that a military response to the Aug. 21 poison gas attack that killed hundreds remained on the table, and were pressing for a U.N. resolution reflecting that in coming days.
"If Assad fails to comply ... we are all agreed, and that includes Russia, that there will be consequences," Secretary of State John Kerry said.
Meanwhile, invitations were going out Monday to top members of the Syrian National Coalition -- the main umbrella opposition group -- for an international conference in New York timed to coincide with next week's U.N. General Assembly meeting, French officials said.
Bolstering the Western-backed SNC is just as crucial to Syria's future as Assad's agreement to give up chemical arms, said French Foreign Minister Laurent Fabius.
"He must understand that there is no military victory, no possible military victory for him," Fabius said. He acknowledged that broad popular support for the rebels has been hampered by fears that Islamic militants are now playing a major role in the 2 1/2-year-old uprising.
In briefing the allies, Kerry was pressing for support for the ambitious agreement that averted threatened U.S. military strikes. It calls for an inventory of Syria's chemical weapons program within one week, with all components of the program out of the country or destroyed by mid-2014.
Those who blame Assad for the chemical attack and supported military strikes say it is up to Assad to uphold his end of any deal.
"It is extremely important that there are no evasions, that there is no cat and mouse game going on over these weapons," said British Foreign Secretary William Hague.
Kerry acknowledged the chemical arms deal would have little immediate effect on the bloodshed in Syria, which has killed more than 100,000 people, but he said full compliance was a key first step.
In Geneva, Pinheiro said the "vast majority" of casualties in Syria's civil war came from conventional weapons like guns and mortars.
The Aug. 21 chemical attack unfolded as a U.N. chemical weapons team was in Syria to investigate earlier reported attacks. After days of delays, the inspectors were allowed access to victims, doctors and others in the Damascus suburbs afflicted by the poison gas. The U.N.'s chief weapons inspector turned over his team's report on Sunday, and the Security Council is due to take it up in a closed session Monday.
The Assad regime insists that the attack was carried out by rebels. The inspection team led by Swedish expert Ake Sellstrom was mandated to report on whether chemical weapons were used and which ones they were, but not on who was responsible.