An official with the global chemical weapons watchdog Thursday said experts had concluded that mustard gas was used in fighting in the Syrian town of Marea in August.
The official from the Hague-based Organization for the Prohibition of Chemical Weapons said it had not yet been determined who used the poison. But for several months, the group has suspected Islamic State of possessing chemical weapons.
The medical charity Doctors Without Borders reported treating four people whose house apparently had been hit by a chemical weapons explosion in August in northern Iraq.
Mustard gas was first used in World War I and has since been outlawed. It is a particularly vicious poison that burns the eyes, skin, lungs and other organs.