The United States believes Islamic State militants likely used mustard agent in an attack on Kurdish forces in Iraq earlier this week, the first indication the militant group has obtained a banned chemical weapon, the Wall Street Journal reported on Thursday.

Citing senior U.S. officials, the Journal said Islamic State could have obtained the mustard agent in Syria, whose government admitted to having large quantities of the blistering agent in 2013, when it agreed to give up its chemical weapons arsenal.

Islamic State could also have obtained the mustard agent in Iraq, the Journal reported.