U.S. military officials in Iraq have issued preliminary confirmation that Islamic State militants used mustard gas in a mortar attack on Kurdish forces in August, a Defense Department official said.

After an Aug. 11 attack that reportedly sickened dozens of Kurdish troops, the Kurds provided U.S. officials with fragments of shells that later tested positive for the presence of "HD, or what is known as sulfur mustard," said Marine Corps Brig. Gen. Kevin J. Killea, chief of staff for Combined Joint Task Force-Operation Inherent Resolve.

The attack occurred in the town of Makhmour in northern Iraq near the front lines of the Kurdish forces' fight against the Islamic State, according to Killea, who briefed reporters at the Pentagon on Friday.

Killea cautioned that this was a "presumptive field test," and further analysis is needed to possibly determine the source of the chemical weapon.

Both Iraq and Syria have in the past maintained stockpiles of chemical weapons, and U.S. officials say it is unclear whether the Islamic State, also known as ISIS and ISIL, has seized any of those weapons.

The HD strain of mustard is listed as a "Schedule I" chemical weapon and is strictly banned under the international treaty known as the Chemical Weapons Convention. When sprayed or released from artillery shells, mustard agents blister skin and can damage lungs if inhaled.

Killea said the potential confirmation of the Islamic State's use of chemical weapons will not necessarily have any impact on U.S. policy.

"We really don't need another reason to hunt down ISIL and kill them wherever we can and whenever we can," he said. "Any indication of the use of a chemical warfare agent, purely from our perspective, reinforces our position that this is an abhorrent group that will kill indiscriminately without any moral or legal code or restraint."