BREAKING: Sick ISIS jihadis launch 'mustard gas attack' on American soldiers at base in Iraq
ISIS are accused of using chemical weapons on an Iraqi base
The missile hit the Qayarrah base, just south of Mosul, where US and Iraqi troops are stationed on Tuesday, according to US officials.
No troops were hurt and so far none have displayed symptoms of exposure to mustard agent, which include skin blistering.
The gas also causes a person's lungs to fill up with fluid, which means that victims eventually stop breathing and die.
But officials have had the shell tested - receiving an initial reading for a chemical agent they believe is mustard.