UN leader Ban Ki-moon on Monday condemned chemical weapon attacks in Syria as a war crime and said UN investigators had indisputable evidence of their use.
Ban called on the Security Council to impose "consequences" for any failure by President Bashar al-Assad to keep to a Russia-US plan to destroy Syria's banned chemical arsenal.
Ban told the 15-nation council that UN investigators have "now confirmed, unequivocally and objectively that chemical weapons have been used in Syria."
"This is a war crime," he added.
"I trust all can join me in condemning this despicable crime," Ban told a closed meeting of the Council, which is divided between supporters and opponents of tough international action against Assad.
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Ban told the council how doctors treated civilians with no external signs of injuries in the streets of Ghouta, east of Damascus, in the hours after the August 21 attack.
The United States, which has threatened a military strike over the chemical arms, estimates that 1,400 people died in the attack.
"The weather conditions that morning were conducive to maximizing the potential impact," Ban said.
"The downward movement of air would have allowed the gas to easily penetrate the basements and lower levels of buildings and other structures where many people were seeking shelter," Ban said.
The UN investigation team said in its report that it had "clear and convincing" evidence that sarin gas was used in Ghouta and that chemical weapons have been used on a "relatively large-scale" in the 30-month-old Syrian conflict.
The meeting came two days after Russia and the United States agreed a plan to destroy Syria's chemical arms within a year.