The Islamic State group will use chemical weapons and are using human shields to defend themselves as Iraqi forces and allies advance into the group's stronghold of the Iraqi city of Mosul, the U.S. claimed Tuesday.

U.S. forces had detected a sulfur mustard agent on fragments of a shell that ISIS had fired against Kurdish forces Oct. 5, Reuters reported Tuesday while quoting an anonymous U.S. official. While the U.S. does not believe that ISIS possesses or has the capability to produce a significant stockpile of advanced chemical weapons, mustard agents can cause blistering of the skin and lungs. The Pentagon reportedly supplied over 50,000 gas masks in Iraq earlier this month.

"Given ISIL's reprehensible behavior and flagrant disregard for international standards and norms, this event is not surprising," another U.S. official told Reuters Tuesday, speaking on condition of anonymity, and using an alternative acronym for ISIS.

A U.S.-led coalition including the Iraqi Army, Kurdish Peshmerga forces and Shiite militias have this week been storming Mosul, once Iraq's second largest city, which fell to ISIS in 2014. The coalition forces have captured at least 9 villages in the outskirts of the city, but are still about 12 to 30 miles away from the city itself. The militants are reportedly going to extreme lengths to maintain their positions.

The U.S. also accused ISIS of using human shields and suicide attacks. About 1.5 million residents are believed to remain in Mosul, making it by ISIS' largest city by far. The terrorist group is expected to defend its grip on the city by any means necessary and prevent the exodus of civilians into nearby Kurdistan.

Pentagon spokesperson Captain Jeff Davis told reporters about the "ugly fight" as well as the latest developments of the operation

"This has been going on for several weeks where we've seen civilians being forcibly detained and their movements being prevented where they can't get out of Mosul. They are being held there against their will," he said.

ISIS’ military tactics, which include killing of prisoners, destruction of cultural sites and forcing female hostages into sex slavery, have been widely criticized by international organizations such as Human Rights Watch. Earlier this year, The United Nations and the Organizations for the Prohibition of Chemical Weapons accused ISIS of using mustard gas in Syria.