Pentagon spokesman Jeff Davis made the remarks on Monday as Iraqi forces have been preparing for a major offensive to liberate Mosul.
The Iraqi troops have recently managed to repel an attack by the extremist group on the town of Qayyarah to the south of Mosul. The forces recaptured Qayyarah from the grip of Daesh in late August.
They also repelled another Daesh attack in the province of Salahuddin, north of Baghdad, early on Monday.
Davis said that "we can fully expect that as this road towards Mosul progresses, ISIL [Daesh] is likely to try to use it again."
He said that Daesh has been packing a "rudimentary" form of mustard into crude, makeshift munitions.
Davis noted that the group's mustard gas is normally a powder mixed with tar, adding it is "not generally in a lethal concentration" and "not anything we would consider significant militarily."
He said that Daesh had already used mustard gas "at least a couple dozen [times]."
The group might have used mustard gas last week in a rocket launched against US soldiers at a base near Qayyarah, according to the Pentagon.
The base is now in the process of being transformed into a logistics hub to support the Mosul offensive.
Davis said that US soldiers based in Iraq, who may be exposed to Daesh's chemical arms, have been trained and received the necessary protective equipment.
"They have practiced it many, many times before they will ever get to a place like Qayyarah West where they could encounter it," he said.
He also said that the US had provided over 50,000 gas masks to the Iraqi military forces.
"We want to make sure that the Iraqi security forces and the Peshmergas (Kurdish fighters) have the ability to detect this and to defend against it."
Violence has plagued the northern and western parts of Iraq ever since Daesh launched its offensive in June 2014, and seized territory. Iraqi army soldiers and fighters from allied Popular Mobilization Units are seeking to win back militant-held regions in joint operations.
The Iraqi army killed some 141 Daesh militants in several military operations across the country on Sunday.
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