A rocket that landed on a military base in northern Iraq where hundreds of US troops are working with Iraqi counterparts may have contained a mustard agent, a military source has said.
No one was hurt and US officials are still testing a residue found on a metal fragment from the rocket.
The official told reporters at the Pentagon that the attack happened on Tuesday at Qayara West air base, a staging area for Iraqi forces preparing for an assault on Mosul, the city that has been the Islamic State's main stronghold in Iraq since 2014.
US officials have been reluctant to say how many American troops are advising and assisting at Qayara West.
The source who described the rocket attack said "hundreds" were there but would not be more precise.
The official said a small group of US soldiers who inspected remnants of the rocket after it exploded found a black, oily substance on a fragment of metal.
An initial test of the suspicious substance showed it contained residue of mustard agent, but a second test was negative.
The fragment has been sent to a laboratory for further testing.
Mustard agent in sufficient quantities and concentration can maim or kill by damaging skin, eyes and airways.
Earlier this month a US general said American warplanes had eliminated a "significant chemical threat" to Iraqi civilians by bombing a complex of buildings near Mosul that IS militants had converted from pharmaceutical manufacturing to chemical weapons production.
Air force Lt Gen Jeffrey Harrigian said the target was an IS headquarters also used to produce lethal chemicals, possibly including chlorine and mustard gas.