Islamic State militants have allegedly used mustard gas against Kurdish forces this week, according to US officials.
In 2013 Bashar Al Assad admitted to having a stockpile of the mustard gas and other chemical weapons. While it was believed that these weapons were destroyed by inspectors, now it appears that the government may have hidden some of the weapons.
With the expansion of IS territory, they now appear to have gotten hold of these weapons and are using them themselves. However, this is only speculation and there is no proof as yet that this is the case.
Kurdish forces fighting IS have reported difficulty breathing following an IS attack, this is one of the symptoms associated with a chemical attack.
On Thursday the Pentagon announced that it was investigating the alleged attack.
Mustard gas is an asphyxiant (affects ability to breathe) that has been banned in war by the UN since 1993. Used extensively during World War One, the gas causes blisters on the skin and lungs. IS have also been accused of using chlorine gas.