Banned chemical weapons have been used on a wide scale in the Syria war and there is clear evidence sarin killed hundreds of people in one major attack, UN inspectors said Monday.
Chemical arms have been used in the 30-month-old conflict "on a relatively large scale," says the report to be released by UN leader Ban Ki-moon.
UN experts, who went to Syria last month, are not allowed to say who carried out the attacks. But they said there is "clear and convincing" evidence that sarin gas killed hundreds of people in an attack on Ghouta near Damascus on August 21.
The attack sparked threats by the United States and other western nations of a military strike on President Bashar al-Assad's forces. The United States says more than 1,400 people died in Ghouta.
Though the military threat has eased after Russia and the United States agreed a plan to put Syrian chemical arms under international control, the UN report will influence what measures are taken to make Assad stick to the plan.
"Surface-to-surface rockets containing the nerve agent sarin were used" in the August 21 attack, said the report.
While Assad blames opposition rebels for the attack, Western nations say only the government has such weapons.
"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 Ghouta, said the first page of the inspectors' report, which was inadvertently leaked by the United Nations.
"This result leaves us with the deepest concern," they added.
The experts said that based on evidence they had found, "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.
A UN-mandated independent commission of inquiry into rights violations in the Syria war announced separately on Monday that it was investigating 14 alleged chemical weapons attacks in Syria.
The UN experts went to Damascus on August 18 to investigate claims that chemical weapons were used at Khan al-Asal, near Aleppo on March 19 and at two other sites, which were named on Monday as Sheik Maqmood and Saraqueb.
The experts were in Damascus when the attack on opposition-held Ghouta was staged on August 21. The team was immediately ordered to concentrate on Ghouta attack and will return later to investigate the other sites.
The detail was in the first page of the report mistakenly made public when it was included in an official picture of UN investigation leader Ake Sellstrom handing over the report to UN leader Ban Ki-moon.
Ban was to give the full report to the UN Security Council on Monday.
While the government, backed by Russia, denies any use of chemical weapons, Ban said on Friday that Assad has carried out "many crimes against humanity" and called for accountability. He did not, however, accuse Assad of using chemical weapons.
The UN Security Council is expected to start negotiations this week on a resolution to back the plan agreed at the weekend by Russia's Foreign Minister Sergei Lavrov and US Secretary of State John Kerry to destroy Syria's chemical stockpile.
After a meeting of their foreign ministers in Paris on Monday, France, the United States and Britain called for a "strong" resolution to put pressure on Assad to stick to the plan.
The United States and France have said that military action in Syria is still possible.
Russia has insisted, however, that it will not agree a UN resolution that includes a threat of force against Assad. Russia and China have vetoed three resolutions since the start of the Syrian uprising in March 2011 that sought to increase pressure on Assad without imposing any sanctions.
Lavrov on Monday warned Monday that western talk of a tough resolution could wreck hopes of convening a Syria peace conference.
"That is another path to wrecking completely the chances of calling the Geneva-2 conference," Lavrov told a press conference in Moscow.