The Islamic State in Iraq and Syria is suspected of firing a crudely-made chemical weapon at a base where hundreds of U.S. troops were operating.

ISIS fired an approximately foot-long rocket at Qayarrah air base in northern Iraq on Tuesday afternoon, a fragment of which tested positive for mustard agent, a U.S. military official told reporters Wednesday.

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After it landed within the base, U.S. troops tested it and received a positive initial reading for mustard agent. A second reading turned up negative, but that could have been due to sun exposure or the passage of time, the official said.

The official added that the rocket landed within 100 yards of the "security perimeter of the base," but did not say how close it got to U.S. troops.

Rocket fragments have been sent to a laboratory for further testing, officials said.

Rocket attacks happen "fairly often" at the base, the official said, but it was the first time ISIS has fired a rocket testing positive for mustard agent at U.S. troops.

The official said after the attack, a U.S. military team went out to inspect the rocket, and noticed "something suspicious." One of the rocket fragments had an oily, tar-like substance.

The substance was not considered to be militarily effective, the official said.

No U.S. troops were hurt or have displayed symptoms of exposure to a mustard agent, the official added.

Symptoms of mustard agent exposure usually come within 12 hours, but there have been no symptoms more than 24 hours later, the official said.

Several hundred U.S. troops are at the air base, preparing it for an Iraqi-led offensive on Mosul, ISIS's stronghold since June 2014. All are equipped with personal protective gear against chemical agents, the official said.

The rocket attack comes after the U.S-led coalition recently bombed a building believed to be a chemical weapons making facility near Mosul.

The official called the potential chemical weapons attack "not surprising," and in line with the things ISIS has been doing to try to "harass us and frighten us."

"We train and equip for a chemical environment," the official said, adding it was a "moderate level of concern."

"This is the kind of things that we expect," the official said.

This story was updated at 5:56 p.m.