WASHINGTON, Aug. 21 (Xinhua) -- The Pentagon said Friday initial tests indicated that the extremist group the 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 U.S. Brigadier General Kevin Killea in a Pentagon briefing.

However, the Pentagon 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 U.S. TV network cnn, U.S. senior officials who spoke on condition of anonymity 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 precursor chemicals, not the same type as those in Syrian President Bashar al-Assad's chemical arsenals, a sign that the group may have mixed chemical weapons on its own.

U.S. intelligence communities have long been concerned that the IS, now in control of a wide swaths of Syrian territory, would find hidden storage of chemical weapons possibly left after conflicts between the Syrian government and anti-government rebels.