UNITED NATIONS » U.N. inspectors said today 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.

The findings represent the first official confirmation by scientific experts that chemical weapons were used in Syria’s civil war, but the report left the key question of who launched the attack unanswered.

The rebels and their U.S. and Western supporters have said the regime of President Bashar Assad was behind the Aug. 21 attack, while the Syrian government and its closest ally, Russia, blame the rebels.

U.S., British and French diplomats said the findings of the U.N. inspectors supported their conclusion that Assad regime was to blame. Russia disagreed.

Secretary of State John Kerry briefed U.S. allies on a broad agreement reached over the weekend with Russia to end Syria’s chemical weapons program, pressing for broad support for the plan that averted U.S. military strikes. Kerry met in Paris with his counterparts from France, Britain, Turkey and Saudi Arabia before seeking a U.N. resolution that would detail how the international community can secure and destroy Syria’s stockpile and precursor chemicals.

As a sign of possible difficulties ahead, Kerry and Russian Foreign Minister Sergey Lavrov sparred today over possible military action if Syria doesn’t abandon its chemical weapons.

And in Geneva, the chairman of a U.N. war crimes panel said it is investigating 14 suspected chemical attacks in Syria, dramatically escalating the stakes. Paulo Sergio Pinheiro said the panel had not pinpointed the chemical used or who is responsible.

Secretary-General Ban Ki-moon presented the U.N. inspectors’ report to a closed meeting of the U.N. Security Council before its release.

"This is a war crime and a grave violation of … international law," Ban told the council in remarks distributed to the press. "The results are overwhelming and indisputable. The facts speak for themselves. … The international community has a responsibility to hold the perpetrators accountable and to ensure that chemical weapons never re-emerge as an instrument of warfare."

The inspectors’ report 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," they added.

Syria initially called for a U.N. investigation of an alleged chemical attack on Khan Al-Assal near Aleppo on March 19, which it blamed on the rebels. But when the U.S., Britain and France reported other alleged attacks, the secretary-general insisted on broadening the investigation, which the Syrians opposed.

After months of negotiations, an agreement was reached in August for U.N. inspectors to go to the sites of Khan Al-Assal and two other alleged attacks. But its mandate was limited, reportedly at Syria’s insistence, 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 report cited a number of facts supporting its conclusion:

— Rocket fragments were found to contain sarin.

— Close to the impact sites, in the area were people were affected, inspectors collected 30 soil and environmental samples — far more than any previous U.N. investigation — and "the environment was found to be contaminated by sarin."

— Blood, urine and hair samples from 34 patients who had signs of poisoning by a chemical compound provided "definitive evidence of exposure to sarin by almost all of the survivors assessed."

— More than 50 interviews with survivors and health care workers "provided ample corroboration of the medical and scientific results."

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. The report said the rockets that hit two of the suburbs, Zamalka and Ein Tarma, were fired from the northwest but didn’t identify the perpetrator.

The inspectors did not provide an exact location on the rockets’ launch site, but Qassioun Mountain, where the Syrian military is known to have bases, is roughly northwest of both suburbs.

The U.S., Britain and France said the findings point to the Assad government as the perpetrator of the attack.

"When you look at the findings carefully, the quantities of toxic gas used, the complexity of the mixes, the nature, and the trajectory of the (gas) carriers, it leaves absolutely no doubt as to the origin of the attack," French Foreign Minister Laurent Fabius told RTL radio this evening. "It reinforces the position of those that have said the regime is guilty."

U.S. Ambassador Samantha Power said "only the regime could have carried out this large-scale attack." She said the quality of the sarin was higher than that used by Iraq’s Saddam Hussein against Iran and there is no evidence the rebels possess the nerve agent.

The inspectors cautioned that the five sites they investigated had been "well traveled by other individuals prior to the arrival of the mission."

"During the time spent at these locations, individuals arrived carrying other suspected munitions indicating that such potential evidence is being moved and possibly manipulated," the report said. The areas were under rebel control, but the report did not elaborate on who the individuals were.

Russia’s U.N. Ambassador Vitaly Churkin called the investigators’ conclusion "deeply disturbing" but stressed that it was too early to jump to conclusions.

"Allegations that in fact it was the opposition who used chemical weapons cannot be simply shrugged off," he said. "Those allegations also need to be very seriously investigated."

Churkin wondered why there were no reports of casualties among opposition fighters if government forces fired rockets filled with sarin to try to oust opposition groups from the area. "Is it theoretically possible to fire five or six rockets and miss your opponent?" he asked.

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.

In the report, chief weapons inspector Ake 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 inspectors’ report for the first time identified the two sites still to be investigated as Sheik Maqsood and Saraqueb.

The report 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.

Kerry and his French and British counterparts worked on a two-pronged approach to Syria. They called for enforceable U.N. benchmarks for eradicating the chemical weapons program and an international conference bolstering the moderate opposition.

An ambitious agreement reached with the Russians 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.

"It has to be strong, it has to be forceful, it has to be real, it has to be accountable, it has to be transparent, it has to be timely. All of those things are critical. And it has to be enforced," Kerry said. "We will not tolerate avoidance or anything less than full compliance by the Assad regime."

Kerry said the agreement "fully commits the United States and Russia to impose measures under Chapter 7 of the U.N. Charter in the event of non-compliance." Chapter 7 resolutions allow for military enforcement.

Russia’s Lavrov said Chapter 7 was the subject of "fierce debate" during the U.S.-Russia talks but stressed that "the final document … doesn’t mention it" and that the Security Council resolution being negotiated will not be under Chapter 7. He said if Syria fails to cooperate, the Security Council can pass an entirely different resolution "which may employ Chapter 7."

Lavrov stressed that ongoing attempts to threaten the use of force against Syria would provoke the opposition and disrupt a chance for peace negotiations in Geneva that the U.S. and Russia have been trying to organize.

Ban urged the Security Council — which has been paralyzed over Syria — to unite and ensure enforcement and compliance with the U.S.-Russia plan.

In London, Syrian Information Minister Omran al-Zoubi said Syria will comply with all Security Council resolutions and will facilitate the mission of the U.N. inspectors in line with the Russian-U.S. agreement. The comments were carried by state-run SANA news agency, which said al-Zoubi made the comments in an interview with Britain-based ITN TV on Sunday.

Meanwhile, invitations were going out today 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.

On the ‘Net:

Report of the U.N. inspectors: http://www.un.org/disarmament/

Associated Press writers John Heilprin in Geneva, Matthew Lee in Paris, Peter Spielmann at the United Nations and Albert Aji in Damascus, Syria, contributed to this report.