American and Iraqi troops are operating in the air base where the shell, either a rocket or artillery shell, landed.
The agent, however, is said to have a “low purity” and “poorly weaponized,” an official said, with a second one calling it “ineffective.”
“It was ISIS that fired at the base, since the terror group has been making mustard agent for some time,” CNN said in its report, citing the Pentagon’s assessment of the situation.
No one is said to have been hurt in the attack while none of the military forces there have been reported as being exposed to the dangerous agent.
The US-led coalition has done little to stop Daesh’s advances in Syria and Iraq. Some analysts have criticized the US-led military campaign, saying the strikes are only meant to benefit US weapons manufacturers.