Reuters International

WASHINGTON (Reuters) - Islamic State forces on Tuesday fired a shell that may have contained a mustard agent onto a military base in northern Iraq used by U.S. and Iraqi troops, CNN reported on Wednesday, citing several U.S. officials.

No U.S. troops were hurt or have displayed symptoms of exposure to a mustard agent, CNN said.

One official said the agent had "low purity" and was "poorly weaponised," CNN reported. A second official called it "ineffective," the network said.

A U.S. defence official said troops had gone out to look at the shell after it landed on the base, which is being used to prepare an attack to retake Mosul from Islamic State, CNN reported.

Two field tests were conducted after the troops saw what they thought was a suspect substance. The first test came back positive but the second test was negative, the network said.

The Pentagon did not immediately respond to a request for comment.

(Writing by Eric Beech; Editing by David Alexander and James Dalgleish)

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