A senior U.S. military officer says preliminary tests show traces of the chemical agent sulfur mustard on mortars that Islamic State militants used to attack Kurdish forces in Iraq.

U.S. Brig. Gen. Kevin Killea, chief of staff for the military operations in Iraq and Syria, says the field testing is not conclusive, so final tests are underway to get the full make-up of the chemicals on the fragments.

U.S. officials have been looking into reports that Islamic State militants used the chemical weapon mustard gas in the August 11 attack in Makhmour. Similar reports surfaced in July.

Killea told Pentagon reporters on Friday that Kurdish forces brought the mortar fragments to U.S. forces for testing, so there may be questions about the chain of custody of the evidence.