Fragments from mortars fired by the Islamic State on Kurdish fighters in northern Iraq earlier this month have tested positive for traces of mustard gas, according to reports.
In a press briefing on Friday at the Pentagon, Brig. Gen. Kevin Killea, chief of staff for operations for the international coalition against the Islamic State, said preliminary tests conducted by the US military on the mortar fragments found traces of the chemical weapons agent sulfur mustard.
According to Reuters, Kurdish Peshmerga fighters collected mortar shells near the town of Makhmur in north-central Iraq on Aug. 11. They then handed the shells over to the US military.
"We were able to take the fragments from some of those mortar rounds and do a field test ... on those fragments, and they showed the presence of HD, or what is known as sulfur mustard," Killea said in the Friday briefing.
Killea cautioned that the initial tests do not provide conclusive evidence of chemical weapons use. He said the fragments are being tested further to confirm the US military's preliminary findings.
"That is a presumptive field test and it is not conclusive, and what those results tell us is merely the presence of that chemical. It doesn't tell us anything more than that," he said, according to CNN.
Sulfur mustard is a Class 1 chemical agent, which means it has few uses outside chemical warfare.
"Kurdish fighters suffered from breathing difficulties" as a result of the attack near Makhmur, said CBS news, citing a German defense department statement.
CBS added that there may be some questions about the "chain of custody" of the fragments, since they were in the hands of Kurdish Peshmerga forces before the US military was able to test them.
The Islamic State has previously been accused of engaging in chemical warfare.