The U.S. military says initial field tests show traces of a chemical agent on mortars used by Islamic State militants to attack Kurdish forces in Iraq.
U.S. Brigadier General Kevin Killea, head of military operations against the Islamic State group in Iraq and Syria, said field tests showed the presence of HD, or what is known as sulfur mustard, a class one chemical agent.
He said the field tests were not conclusive and that final tests are underway to make a definitive assessment.
U.S. officials have been looking into reports IS militants may have used mustard gas against Kurdish Peshmerga fighters this month in Makhmour, Iraq.
General Killea said Kurdish forces brought mortar round fragments from that attack to the U.S. military to test for traces of chemical weapons, but he cautioned there are concerns about the Peshmerga providing the U.S. with misleading evidence.
It is unclear where IS militants would have obtained the weapon.