If only Obama drew a red line.

WASHINGTON, Aug 13 (Reuters) – 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.

“We have credible information that the agent used in the attack was mustard,” a senior U.S. official told the Journal.

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, the newspaper reported.

“That makes the most sense,” the Journal quoted a senior U.S. official as saying about the possibility that Islamic State obtained the mustard agent in Syria.

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

The newspaper did not specify where or when the attack occurred, or whether it caused casualties.