The United States expects Islamic State (ISIS) to use crude chemical weapons as it tries to repel an Iraqi-led offensive on the city of Mosul, U.S. officials said Tuesday, according to Reuters.
They added, however, that the group's technical ability to develop such weapons is highly limited.
U.S. forces have begun to regularly collect shell fragments to test for possible chemical agents, given Islamic State's use of mustard agent in the months before Monday's launch of the Mosul offensive, one official said.
ISIS is known to have used mustard gas on several occasions in both Syria and Iraq.
In September, the group was suspected of firing a shell containing mustard agent that landed at the Qayarrah air base in Iraq, where American and Iraqi troops are operating.
In a previously undisclosed incident, reported Reuters, U.S. forces confirmed the presence of a sulfur mustard agent on Islamic State munition fragments on October 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 official told Reuters, speaking on condition of anonymity, and using an alternative acronym for Islamic State.
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.
Despite the incidents of chemical weapons, U.S. officials do not believe ISIS 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.