The Islamic State is "dead set" on using chemical weapons attacks, including sulfur-mustard gas, to endanger U.S. troops and blunt or delay the long-planned offensive to retake Mosul in northwestern Iraq, a Pentagon spokesman said Monday.
"I think we can fully expect, as this road toward Mosul progresses, ISIL is likely to try to use it again," Navy Capt. Jeff Davis said, using another acronym for the Islamic State of Iraq and Syria, or ISIS. "They are dead set on it."
Last week, ISIS fighters fired an artillery shell near U.S. troops at the Qayyarah West airfield, about 40 miles southeast of Mosul, that was initially suspected of having traces of sulfur-mustard blistering agent. There were no deaths or injuries in the incident.
In a briefing from Baghdad to the Pentagon last Friday, Air Force Col. John Dorrian, the spokesman for Combined Joint Task Force-Operation Inherent Resolve, said that first test of an oily substance on shell fragments was positive, a second test was negative, and a third was inconclusive.
"We have no conclusive evidence" that mustard gas was used, Dorrian said. He said more tests were being conducted.
However, Kurdish peshmerga forces participating in the "shaping operations" for the Mosul offensive said last year that ISIS fired mortar shells suspected of containing mustard gas at their positions about 20 miles east of the Qayyarah airfield. ISIS is also suspected of using chlorine gas in Syria.
Earlier this month, U.S. and coalition aircraft carried out strikes against a former pharmaceutical factory in Mosul that ISIS was suspected of having turned into a chemical weapons complex.
At the Pentagon, Davis said ISIS "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."
U.S. troops in Iraq have access to gas masks and Mission Oriented Protective Posture (MOPP) gear to protect against chemical attacks.
In the 2003 invasion of Iraq, troops carried masks and MOPP suits with them at all times and frequently had to don them as alarms went off on the possibility that chemical weapons were in the area.
Later U.S. inspections and reports found that Iraq had stopped producing and stockpiling weapons of mass destruction before the invasion.
"We fully recognize that this is something that ISIL has done before," Davis said of the possibility of chemical attacks. "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."
"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," he said.
In addition to U.S. troops having access to gas masks and MOPP gear, Davis said the U.S. has distributed more than 50,000 kits of personal protective gear for Iraqi and Kurdish forces.
In the Mosul offensive, American advisers are expected to move closer to the battlefront. The Defense Department has authorized U.S. commanders to place advisers with the Iraqi Security Forces and Kurdish peshmerga at the battalion level.
In his briefing last Friday, Dorrian said eight to 12 brigades of the Iraqi Security Forces were "ready to go" against Mosul, where ISIS has had nearly two years to build up defenses. The U.S. estimates that the group "no longer is able to mass enough forces to stop the advance" on the city, and its fighters are experiencing "flagging morale" from the loss of territory and the unrelenting coalition airstrikes, Dorrian said.
U.S. airstrikes recently destroyed an estimated 29 ISIS boats on the Tigris River and also blew up a bridge over which the group's vehicles were attempting to escape, he said.
To defend Mosul, ISIS has built "intricate defenses," including elaborate tunnel networks and interconnected layers of improvised explosive devices along likely "avenues of approach" to the city, Dorrian said.
The U.S. has also seen reports that ISIS has dug trenches and filled them with oil to be set on fire once the offensive begins. "They've built a hell on earth around themselves," he said.
-- Editor's Note: This story has been updated to correct the spokesman's service.
-- Richard Sisk can be reached at Richard.Sisk@Military.com.