A rocket or artillery shell fired near U.S. troops in Iraq last week may or may not turn out to have been laced with mustard agent. But the Pentagon says it's only a matter of time before U.S. forces in Iraq or Syria come under chemical attack from the Islamic State.

Tests on the fragments of the munition that fell near the Qayyarah West base south of Mosul remain inconclusive, yet Pentagon spokesman Capt. Jeff Davis said that as Islamic State fighters in Mosul become more desperate, they are likely to resort to rudimentary chemical weapons.

"This is something ISIL has done before," Davis told reporters at the Pentagon. "They have done it many times, at least a couple of dozen that we know of, where they have launched crude, makeshift munitions that are filled with this mustard agent."

The U.S. military says a shell was recovered some distance away from where about 200 American troops are preparing the Iraq base to be used as a staging area for the future Mosul offensive, and that the fragments of the munition had marks that typically indicate presence of mustard agent in powder form that is bound up with oil.

"It's not generally in a lethal concentration," Davis said. "It's more of an irritant than anything else."

Davis said all U.S. troops in Iraq and Syria are trained and equipped to deal with chemical agents on the battlefield, but with the assault on the Islamic State's stronghold in Mosul looming, the U.S. has also provided 50,000 gas masks to Iraqi and Peshmerga forces.

"I think we can fully expect as this road toward Mosul progresses, ISIL is likely to try to use it again," Davis said. "So, this is something that could happen."

Because the chemical weapons used by the Islamic State are so unsophisticated, they are not seen by the U.S. as militarily significant. Davis said nevertheless it's further evidence that the Islamic State knows no boundaries when it comes to conduct on the battlefield.