U.S. troops in Iraq are facing the threat of more chemical weapons attacks as Islamic State militants prepare to defend against a final invasion of Mosul, a defense official said.

The dire warning from the Pentagon comes almost one week after a shell that tested positive for "mustard agent" landed inside the wire at Qayara West, an Iraqi-controlled air base where several hundred U.S. forces are helping Iraqi Army units build a staging area for the upcoming offensive into Mosul.

"I think we can fully expect, as this road toward Mosul progresses, [the Islamic State group] is likely to try to use it again, said Navy Capt. Jeff Davis, a Pentagon spokesman.

"They are dead set on it. They would love to use chemical weapons against us and against the Iraqis as they move forward and we are making every effort to make sure we are ready for it," Davis said.

U.S. troops in Iraq have access to gas masks and full-body protective gear in the event of a chemical weapons attack. The daily procedures for storing, carrying and using that gear varies depending on force-protection levels set by local commanders, a defense official said.

No one was killed or injured in the Sept. 20 attack, which was the first time a mustard attack in Iraq struck in close proximity to U.S. forces. The shell fragment had an "oily substance" that initially tested positive as a "mustard" agent, or sulfur mustard, which can cause severe skin burns or lung damage if inhaled, defense officials said.

U.S. military officials say the mustard agent was not in a gas state, which would be a far more lethal form. The Islamic State group, also known as ISIS or ISIL, makes the mustard agent in laboratories inside its territory in Iraq and Syria, and has used it in numerous attacks during the past two years.

The ISIS-made product is in a crude form, typically a powered form mix with oil and strapped to conventional munitions, that U.S. military officials say it is not "militarily significant," Davis said.

"We fully recognize that this is something that ISIL has done before. 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," Davis said.

"That is not something we view as militarily significant but obviously it is further evidence that ISIL knows no boundaries when it comes to their conduct on the battlefield," Davis said.

Widely used in Europe's trench warfare during World War One, sulfur mustard is prohibited under the 1993 Chemical Weapons Convention, an arms control treaty signed by nearly every nation in the world.

In further response to the ISIS threat of chemical weapons, the U.S. military has provided more than 50,000 kits of personal protective gear for Iraqi and Kurdish forces, Davis said.

During the past two years, ISIS militants have deployed mustard in numerous attacks, often on Kurdish forces. Occasionally those attacks have sickened Kurdish forces.

U.S and Iraqi officials say an invasion of Mosul, Iraq's second-largest city, is imminent and could begin in October.