The findings represent the first official confirmation by scientific experts that chemical weapons were used in the Syrian conflict, but the first page of the report, seen by The Associated Press, left the key question of who launched the attack unanswered.
The report came as the chairman of a U.N. war crimes panel said it is investigating 14 suspected chemical attacks in Syria, dramatically escalating the stakes after diplomatic breakthroughs that saw the Syrian government agree to dismantle its chemical weapons program.
Secretary-General Ban Ki-moon was scheduled to present the U.N. inspectors' report to the U.N. Security Council later Monday morning.
The inspectors said "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, also against civilians, including children, on a relatively large scale," the inspectors said in their report to Ban.
"This result leaves us with the deepest concern," the inspectors said.
The inspectors were mandated to report on whether chemical weapons were used and if so which ones — not on who was responsible.
The rebels and their Western and Arab supporters blame President Bashar Assad's regime for the attack in the rebel-controlled area of Ghouta. The Assad regime insists that the attack was carried out by rebels. The U.N. report mentions the Ghouta areas of Ein Tarma, Moadamiyeh and Zamalka, all of which were featured in the videos of victims that emerged shortly after the attack.
The Aug. 21 chemical attack unfolded as the U.N. inspection 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.
Chief weapons inspector Ake Sellstrom handed over the report to the secretary-general on Sunday amid a flurry of diplomatic activity aimed at getting Syria to put its just-acknowledged stockpile of chemical weapons and chemical precursors under international control for destruction.
In the report, Sellstrom said the team was issuing the findings on the Ghouta attacks "without prejudice" to its continuing investigation and final report on the alleged use of chemical weapons in three other areas. The letter said it hoped to produce that report as soon as possible.
Under an Aug. 13 agreement between the U.N. and the Syrian government, Sellstrom's team was scheduled to investigate an alleged chemical weapons attack on March 19 on the village of Khan al Assal outside Aleppo and alleged attacks on two other sites which were kept secret for security reasons.
The letter for the first time identified the two sites still to be investigated as Sheik Maqsood and Saraqueb.
It also thanked the four laboratories designated by the Office for the Prohibition of Chemical Weapons (OPCW) to examine the samples from Syria, disclosing their locations for the first time — in Finland, Germany, Sweden and Switzerland.
In Geneva, the chairman of a U.N. war crimes panel, Paulo Sergio Pinheiro, said his panel has not pinpointed the chemical used in the 14 suspected chemical attacks it said Monday it was investigating.
Pinheiro also said the panel believes 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 after laying out with his French and British counterparts a two-pronged approach in Syria. They called for enforceable U.N. benchmarks for eradicating the chemical weapons program and an international conference bolstering the moderate opposition.
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.
France and the U.S. insisted that a military response to the Aug. 21 attack 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," Kerry said.
But Russia's foreign minister, Sergey Lavrov, said ongoing attempts to threaten the use of force against Syria would provoke the opposition and disrupt a chance for negotiations.
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 ½-year-old uprising that has left more than 100,000 dead.
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.
In Geneva, Pinheiro said the "vast majority" of casualties in Syria's civil war came from conventional weapons like guns and mortars.
Associated Press writers John Heilprin in Geneva and Matthew Lee in Paris contributed to this report.
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