Chemical weapons have been used by both Daesh and (on a much bigger scale) the Assad government in the Syrian civil war. The verdict is from a final report by the Joint Investigative Mechanism (JIM) of the United Nations and the Organisation for the Prohibition of Chemical Weapons.
The Syrian government promised in 2013 to give up its chemical weapons under a deal negotiated by the Russian government, but has continued to use them. The committee recorded four uses of VX nerve gas, 13 uses of sarin, 12 of mustard gas, 41 of chlorine and 61 of other chemical agents, and named Daesh as having used mustard gas. However, Syria’s civil war remains overlaid, not by any consistent drive to apply international law, which bans the use of chemical weapons, but by delicate politicking between the USA, Russia, and Turkey, all of them with aims at odds with the others. Russia will almost certainly block action against Assad over chemical weapons.
The Kurdish led drive to expel Daesh from the city of Manbij in northern Syria has met a severe backlash from Turkey. Turkish forces have begun incursions into Kurdish-held territory in Syria, with a promise to “deal with” the Kurdish forces as they would with Daesh. The Kurdish YPG, with the US-backed Syrian Democratic Forces (SDF) coalition, have successfully driven Daesh from large swathes of Northern Syria. The US has called on Turkey to stop attacks on the SDF and YPG, whilst reassuring Turkey that the SDF have largely withdrawn from the east of the Euphrates River and do not intend to go closer to the Turkish border.
The US has continued to provide air support to the YPG and SDF. Once that stops, it is likely that Russian planes will enter the airspace with the aim of stopping Syrian opposition forces from attacking government-held areas.