Daesh militants likely used mustard agent against Kurdish forces in Iraq this week, senior US officials said, in the first indication the militant group has obtained banned chemicals, according to the Wall Street Journal.

The officials said the so-called Islamic State could have obtained the mustard agent in Syria, whose government admitted to having large quantities in 2013 when it agreed to give up its chemical-weapons arsenal.

The use of mustard agent would mark an upgrade in the so-called Islamic State’s battlefield capabilities, and a worrisome one given US intelligence fears about hidden caches of chemical weapons in Syria, where Daesh controls wide swaths of territory.

It raises new questions about the evolving threat posed by the so-called Islamic State and the ability of US allies on the ground to combat it. Front line Kurdish, Iraqi and moderate Syrian forces say they aren’t getting enough US support now to counter the so-called Islamic State’s conventional capabilities.

Officials say these forces may need specialized equipment and training to help protect them against unconventional weapons if they become a fixture on the battlefield.

The attack in question took place late Wednesday, about 40 miles southwest of Erbil in northern Iraq. A German Defense Ministry spokesman said about 60 Peshmerga fighters, who help protect Kurdish areas in northern Iraq, were reported to have suffered injuries to their throats consistent with a chemical attack while fighting the so-called Islamic State.

U.S. intelligence agencies are still investigating the source and how it could have been delivered this week on the battlefield, officials said.