U.S. officials are now reporting that ISIS fighters have attacked U.S. and Iraqi forces with a “crude” chemical weapon that may have been a mustard agent.

The U.S base outside Qayyarah, 25 miles south of Mosul, was struck by a rocket and traces of a “mustard agent” were believed to be present, the official said. The attack was first reported by CNN.

A Pentagon official told reporters that a “tar-like black oily substance” was found on the shell, which landed within the base hundreds of yards from U.S. forces. An initial test for the agent was positive, but “could be false,” the official said. The second test was negative, possibly because the shell had been exposed to the elements.

No U.S. troops were injured, though some went through chemical decontamination. The nature of the weapon is still under investigation.

The shell was categorized by officials as either a rocket or artillery shell. After it landed on the base, just south of Mosul, US troops tested it and received an initial reading for a chemical agent they believe is mustard.

…One official said the agent had “low purity” and was “poorly weaponized.” A second official called it “ineffective.”

Recently, a known ISIS chemical-weapons manufacturing plant was destroyed by U.S.-led forces.

…Footage of air strikes on Monday showed 12 aircraft from the US-led coalition bombarding a large industrial site in Mosul.

Huge explosions could be seen hitting multiple buildings and storage units, leaving them in flames with large plumes of smoke rising into the air.

…Lieutenant General Jeffrey Harrigian, from US Air Forces Central Command said the facility posed a “significant chemical threat to innocent Iraqis”.

“Intelligence had indicated that Daesh [Isis] converted a pharmaceutical plant complex into a chemical weapons productions capability,” he added, saying 50 individual targets were hit.

Mustard gas (a.k.a. sulfur mustard) is a powerful irritant and blistering agent that damages the skin, eyes, and respiratory tract. It can also damage the DNA, especially in the bone marrow. Exposure to it is usually not fatal, but respiratory problems, chronic blood problems, and scars from chemical burns can result.