Syrian troops and Islamic State militants carried out chemical weapon attacks in the war-torn country during 2014 and 2015, say the United Nations and a global chemical weapons watchdog group. Syrian President Bashar al-Assad’s air force was responsible for at least two chlorine gas attacks against opposition-controlled towns in April 2014 and September 2015, say the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW). Chlorine’s use as a weapon is prohibited under the Chemical Weapons Convention, an international treaty that Syria joined in 2013. That year, Syria agreed to dismantle and destroy its stockpile of chemical weapons. The UN-OPCW team also concludes that the Islamic State was “the only entity with the ability, capability, motive, and the means to use sulfur mustard” in Aleppo in August 2015. Also known as mustard gas, this material too is banned as a chemical warfare agent. The UN-OPCW team says that since December 2015, it received more than 130 new reports of chemical weapons use in Syria. It says 41 claim the use of chlorine, 13 sarin, 12 mustard gas, and four VX nerve gas, and 61 of the allegations involve other toxic chemicals.