ISIS flag (illustration)
ISIS flag (illustration)Reuters

Islamic State (ISIS) jihadists are "dead set" on using chemical arms and are likely to try them again as Iraqi forces advance on Mosul, a Pentagon spokesman warned on Monday, Reuters reported.

The comments come days after ISIS jihadists fired a shell containing mustard agent that landed at the Qayarrah air base in Iraq, where American and Iraqi troops are operating.

That incident followed several reported cases of ISIS using mustard agent in both Syria and Iraq. CIA chief John Brennan has warned that the jihadists have used chemical weapons and have the capability to make small quantities of chlorine and mustard gas.

The rocket fired last Tuesday landed in an unpopulated area near the base. No one was hurt in the attack.

The shell that was fired initially tested positive for a mustard agent, but two subsequent tests have been inconclusive and the device is undergoing further tests, Navy Captain Jeff Davis, a Pentagon spokesman, told reporters on Monday, according to Reuters.

"We fully recognize this is something that ISIL has done before. They've done it many times, at least a couple dozen that we know of where they have launched crude makeshift munitions that are filled with this mustard agent," Davis said using an alternative acronym for the group.

An air strike by the U.S.-led military coalition destroyed an ISIS chemical weapons factory on Friday near Qayyara, the second attack against a chemical arms facility this month.

Davis said the group’s ability to weaponize mustard agent has been rudimentary. The group typically uses a chemical powder bound together with oil, which leaves behind a telltale oil trace.

"It's not generally in a lethal concentration. It's more of an irritant than anything else, but again, not something we view as militarily significant," he said, according to Reuters, noting that the gas form of mustard agent used in the First World War was far more lethal.

Even though Islamic State has not perfected the ability to weaponize chemicals, U.S. and Iraqi forces still have to be prepared for a chemical attack, Davis warned.

"We recognize this is real. They're dead set on it. They would love to be able to use chemical weapons against us, against the Iraqis as they move forward," he added. "We are making every effort to make sure that we're ready for it."

Davis said the United States has provided more than 50,000 gas masks to Iraq, with about 40,000 going to Iraqi security forces.

In June of 2014, ISIS seized a weapons complex thought to have held hundreds of tons of lethal sarin and mustard gasses: the al-Muthanna complex, located 60 miles north of Baghdad, which was a central base of former Iraqi dictator Saddam Hussein's chemical weapons program.

There has also been some speculation that ISIS got its hands on chemical stockpiles that belonged to former Libyan dictator Muammar Qaddafi.