Pentagon officials have confirmed that they are investigating reports of Islamic terrorist group ISIS using chemical weapons against Kurdish fighters in northern Iraq.

The officials were quick to emphasize that it was likely a small amount and in a weak concentration, but it still would confirm the terrorists have found at least one cache of Syrian chemical weapons.

A senior U.S. official said that after a barrage fired at a Peshmerga unit last week, there were  “wounds consistent with a blister-producing agent.”

“We continue to monitor these reports closely, and would further stress that any use of chemicals or biological material as a weapon is completely inconsistent with international standards and norms regarding such capabilities,” Alistair Baskey, spokesman for the White House’s National Security Council, said in a statement.

Peshmerga Brig. Gen. Sirwan Barzani told CNN they suspect the agent used was mustard gas because some troops also suffered breathing difficulties beyond the skin blistering.

ISIS had been previously accused by monitoring groups of using chlorine based chemical weapons against Kurdish forces.

“We continue to take these and all allegations of chemical weapons use very seriously. As in previous instances of alleged ISIL use of chemicals as weapons, we are aware of the reports and are seeking additional information. We continue to monitor these reports closely, and would further stress that use of any chemicals or biological material as a weapon is completely inconsistent with international standards and norms regarding such capabilities,” Blake Narendra, a spokesperson for the State Department’s Arms Control, Verification and Compliance Bureau, told reporters.

Mustard gas is only deadly in large quantities but can cause painful burns and blisters that could render an enemy immobile from pain.