Washington, Aug 22 (IANS): The Pentagon said on Friday that initial tests indicated that the militant group Islamic State (IS) may have used a chemical weapon against Kurdish forces in Iraq earlier this month.

Fragments from some mortar rounds fired by the IS near Makhmour in Iraq tested positive for sulfur mustard agent, said US Brigadier General Kevin Killea in a Pentagon briefing, Xinhua reported.

The Pentagon, however, cautioned that a definite assessment could only be made after more testing.

"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," said Killea.

According to the US television network CNN, US senior officials said test results from another IS attack in Syria three weeks ago also confirmed the group used a mustard agent as a weapon.

Citing one official, CNN reported that the mustard agent used in Syria by the IS was more likely to be precursor chemicals -- not the same type as those in Syrian President Bashar al-Assad's chemical arsenal -- a sign that the group may have mixed chemical weapons on its own.