American officials fear that Islamic State jihadists will use crudely-designed chemical weapons, primarily packed with a sulfur mustard gas agent, to defend their de-facto capital of Mosul in Iraq from advancing Iraqi forces, Kurdish troops and Iran-allied Shiite militias, backed by U.S. troops.

Reuters quoted the Iraq chief for the International Organization for Migration (IOM), Thomas Weiss as indicating that “he expected Islamic State militants to use Mosul residents as human shields and lent his voice to concerns about the dangers of chemical agents.”

“The IOM had not managed to procure many gas masks yet, despite those risks, Weiss said from Baghdad,” added the news outlet.

“We also fear, and there has been some evidence that ISIL might be using chemical weapons. Children, the elderly, disabled, will be particularly vulnerable,” Weiss declared.

Fox News reports that “around 700,000 civilians are believed to be living in Mosul,” Iraq’s second largest city and ISIS’s last major stronghold in the country.

ISIS terrorists in northern Iraq, where Mosul in located, have already targeted U.S. troops, Kurdish Peshmerga forces, and civilians with chemical weapons.

In late September, The Telegraph learned that in anticipation of the Mosul operation, which officially began Monday after months of preparation, ISIS terrorists “rigged one of Iraq’s largest chemical plants with explosives.”

The newspaper noted that ISIS was planning to blow up the “Misraq chemical plant and sulphur mine [which] lies roughly 30 miles south of the city [of Mosul] and six miles north of where several hundred US troops are stationed alongside the Iraqi army at Qayyarah airbase.”

On September 20, U.S. troops at the Qayyarah base were targeted by ISIS chemical weapons, marking the first recorded chemical attack on the United States since the jihadist group conquered swaths of Iraq in 2014. No one was killed or injured in the assault.

Meanwhile, unnamed U.S. officials told Reuters that the jihadi group’s chemical weapon capabilities are highly limited while acknowledging that they expect ISIS to use chemical agents to defend Mosul.

Reuters reports:

In a previously undisclosed incident, U.S. forces confirmed the presence of a sulfur mustard agent on Islamic State munition fragments on Oct. 5, a second official said. The Islamic State had targeted local forces, not U.S. or coalition troops.

“Given ISIL’s reprehensible behavior and flagrant disregard for international standards and norms, this event is not surprising,” the second U.S. official told Reuters on condition of anonymity.

On Sept. 26, the Pentagon conceded that ISIS terrorists were determined to use chemical weapons in an attack as U.S.-backed troops advance on Mosul.

“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,” U.S. Navy Capt. Jeff Davis, a Pentagon spokesman, told reporters.

Echoing the U.S. officials who recently spoke to Reuters, Davis downplayed the lethality of the jihadist group’s chemical weapons capability.

Reuters points out:

U.S. officials do not believe Islamic State has been successful so far at developing chemical weapons with particularly lethal effects, meaning that conventional weapons are still the most dangerous threat for advancing Iraqi and Kurdish forces – and any foreign advisers who get close enough.

Sulfur mustard agents can cause blistering on exposed skin and lungs. At low doses, however, that would not be deadly.

Unnamed U.S. officials told Fox News:

ISIS has used mustard agent on “multiple” occasions against Kurdish Peshmerga forces… adding that nobody has been seriously hurt or killed so far.

Mustard agent gives victims symptoms similar to “poison ivy,” according to one official who said ISIS use of the chemical is more like a “weapon of mass disruption.”

That being said, ISIS launched chemical attacks in Iraq in March that exposed a child to an agent or agents that proved to be deadly and wounded hundreds of people.

In June, Breitbart News learned from the Kurdish Peshmerga that ISIS had increased their chemical attacks against their colleagues.

Currently, more than 5,000 U.S. troops have been deployed to Iraq.

Military.com reported earlier this week:

To defeat the ISIS defenders, the Iraqi Security Forces have assembled 12 U.S.-trained brigades which, combined with Kurdish forces and Shiite Popular Mobilization Units [PMU] linked to Iran, are expected to make up an attacking force of 20,000 to 30,000…

U.S. military and civilian officials have estimated that ISIS has about 3,000 to 5,000 fighters to defend the city

Troops from the U.S.-designated terrorist group Hezbollah, an Iranian narco-terrorist proxy, are also believed to be near the frontline of the Mosul operation along with other Shiite militiamen “with horrific records of sectarian killings” and deadly attacks against American troops, The New Yorker reports.