A U.S. military official says Islamic State is suspected of firing a rocket or artillery shell that landed on a military base in northern Iraq and may have contained a mustard agent.
In remarks to reporters at the Pentagon, an official said no one was hurt in Tuesday's attack on the Qayyarah West air base, but several hundred U.S. troops were on the base at the time.
The official said the rocket fell within the security perimeter of the base, but called the attack "militarily insignificant" and said it had not affected operations of the mission in any way.
The official said "hundreds" of U.S. troops are at the base, but would not give details.
Qayyarah West base is being used as a staging area for Iraqi forces preparing an assault on Mosul, the northern city that has been an Islamic State (IS) stronghold for the past two years.
The official said a black, oily substance was found on a piece of ordnance that fell on the grounds of the base. Two field tests were conducted; the first indicated the substance might be a mustard agent, but a second test was negative.
The official said a third fragment has been sent to a lab for more testing.
Two to four troops who inspected the fragment underwent decontamination after examining the shell, as a precautionary measure. None of the troops have reported any symptoms of exposure.
The official said there have been a number of instances this year in which IS has used chemical agents against Iraqi, Kurdish and Syrian forces. The official said this was the first time such an agent had come close to U.S. troops.
U.S. forces last week destroyed a facility near Mosul that was suspected of being used to create chemical weapons.
Carla Babb contributed to this story.