U.S. troops in Iraq are facing the threat of more chemical weapons attacks as Islamic State militants prepare to defend against a final invasion of Mosul, a defense official said.

The dire warning from the Pentagon comes almost one week after a shell that tested positive for “mustard agent” landed inside the wire at Qayara West, an Iraqi-controlled air base where several hundred U.S. forces are helping Iraqi Army units build a staging area for the upcoming offensive into Mosul.

“I think we can fully expect, as this road toward Mosul progresses, [the Islamic State group] is likely to try to use it again, said Navy Capt. Jeff Davis, a Pentagon spokesman.

“They are dead set on it. They would love to use chemical weapons against us and against the Iraqis as they move forward and we are making every effort to make sure we are ready for it,” Davis said.