ISIL has no inhibitions about using WMD
The Obama administration has been unusually quite about this usage of WMD. While Obama talked about a phantom "red line" for Syria's use of such weapons, his bluff got called and he backed off. I suspect the reason he has not make much of ISIL's use is that it would require him to respond with US troops and he seems loathed to do so.
More than one-third of the chemical attacks, which include chlorine and sulfur mustard agents, have come in and around Mosul, the group’s stronghold in Iraq.