New York - Chemical weapons investigators concluded "with a high degree of confidence" that chlorine gas was used as a weapon against three opposition-controlled villages in Syria last year, affecting between 350 and 500 people and killing 13, according to a report obtained on Tuesday by The Associated Press.The third report by a fact-finding mission from the Organisation for the Prohibition of Chemical Weapons didn't apportion blame but said 32 of the 37 people interviewed "saw or heard the sound of a helicopter over the village at the time of the attack with barrel bombs containing toxic chemicals."The investigators said 26 people heard the distinctive "whistling" sound of the falling barrel bombs containing toxic chemicals and 16 visited the impact sites and saw the bombs or their remnants.They said 29 people smelled "the distinctive odour of the gas cloud" released after the bombs hit the ground, mainly describing it "as intense, chlorine-like, similar to cleaning material used to clean toilets, but much stronger".International peace and securityThe report includes a description of 142 videos and 189 pieces of material obtained by the investigators as well as photos of impact sites and the inner chlorine cylinder from a barrel bomb.The mission was established by the OPCW on April 29 to establish the facts surrounding allegations of the use of chlorine "for hostile purposes" in Syria. Chlorine gas is readily available and is used in industry around the world, but can also be used as a weapon.The UN Security Council has been intensely involved in the issue of alleged chemical weapons use in Syria. After an August 2013 sarin gas attack near Damascus in which the US says more than 1 400 people were killed, the Security Council unanimously adopted a resolution backed by the US and Russia on 27 September 2013, ordering Syria's chemical weapons stockpile to be destroyed.The UN investigators could not find enough evidence to assess blame for the sarin attack. Syria's declared chemical weapons stockpiles have since been destroyed under international supervision, but questions remain about whether it may still be hiding deadly chemical agents.Chlorine gas is not listed as a chemical weapon. But eight council members, including the United States, said in a 30 December letter accompanying the OPCW report that the 2013 resolution also states that any use of chemical weapons threatens international peace and security and must be condemned.Conventional weaponsThe 15 council members discussed the fact-finding mission's report behind closed doors on Tuesday, and diplomats said the US and other Western nations who signed the letter along with Jordan urged Security Council action in response to the findings.But Russia, Syria's closest ally, insisted that the report on chlorine attacks was an issue for the OPCW, which polices the Chemical Weapons Convention, the diplomats said, speaking on condition of anonymity because consultations were private.Syria's Deputy Foreign Minister Faysal Mekdad told an OPCW meeting on Dec. 1 that his government has never used chemical weapons or chlorine gas during the country's four-year civil war, which has claimed over 200 000 lives and displaced one third of the country's population. He said terror groups "have used chlorine gas in several of the regions of Syria and Iraq."But US Ambassador Samantha Power tweeted that "only Syrian regime uses [helicopters]". She also tweeted that the Syrian "Regime must be shown it is not enough to destroy declared CW [chemical weapons]; must stop dropping chemical-laden explosives on civilians."The investigators interviewed 14 people from the village of Talmenes in Idlib governorate about barrel bomb attacks on 21 April and 24 April. At two houses that were hit, a 7-year-old boy, a teenage girl, and the matriarch of a family died from exposure to chlorine gas, they said. Domestic animals including cows, goats and sheep also died at both houses.Fourteen people from the village of Al Tamanah, also in Idlib, were interviewed by the mission's investigators about five incidents in April and May — all but one at night. Eight members of two families who had sought refuge in the village died shortly after separate attacks involving the toxic chemical, the report said.Investigators said they interviewed nine people from Kafr Zita in Hama Governorate in northern Syria and were told that the village had been the target of hundreds of attacks with conventional weapons and 17 attacks using toxic chemicals between April and August.