The shell, which landed on Tuesday, was immediately tested, with initial readings revealing that it contained the mustard agent.
It’d be the first chemical weapons assault on coalition forces in Iraq, if supported.
The US troops are working with the Iraqi forces ahead of an offensive that will be launched on Isis stronghold, Mosul.
Gen Dunford told the US Senate armed services committee on Thursday that the rocket had contained a “sulphur-mustard blister agent”.
The shell was tested in the field twice by coalition troops, the officials said.
Following the attack, a small team of US personnel conducting a routine inspection for unexploded ordnance in the perimeter around the base came across a small shell fragment that contained what the official described as an oily “tar-like” residue, which sometimes indicates mustard agent.
The Islamic State (ISIL, also known as Daesh), a designated terrorist organization that is outlawed in Russian Federation and the United States, captured Mosul in 2014, along with a number of other northern and western Iraqi settlements.
The US official also said that the shell was “militarily ineffective”.
Still, the troops who handled the fragments went through a standard decontamination procedure. Islamic State has used weapons with a mustard agent about two dozen times.
USA officials emphasized that mustard agent is relatively easy to produce, and they continue to hit suspected manufacturing sites when they find them.
WASHINGTON-The U.S. military is requesting authority to send up to 500 new troops to Iraq ahead of a much-anticipated campaign to take back Mosul from Islamic State, according to U.S. officials, adding to an expanding American presence in the country.
Accurate numbers for the population remaining in the city are hard to come by but the United Nations and other officials have said up to one million civilians may still be living under ISIL rule in the Mosul area. Indeed one of the officials quoted by CNN have said that they “had expected” that IS might try to use chemical weapons.
The spokesman for the US-led coalition against ISIL, Col John Dorrian, said that the coalition carried out “a very successful strike that eliminated a significant number of fighters who were trying to flee toward Hawija”, to the east of Shirqat, which is under ISIL control.