US troops in Iraq
ISIS may have fired mustard agent rockets at US troops in Iraq, defence official Pictured: U.S soldiers gather in the town of Gwer northern Iraq August 31, 2016.Reuters

American and Iraqi troops were attacked at an airfield near the town of Qayyarah, Iraq, with a rocket that may have carried mustard agent, a US defence official said on Wednesday.

None of the soldiers were injured in the blast near the airfield and neither did they display any signs of exposure to the chemical agent. However, the troops who had dealt with the rocket went through a decontamination process.

The US troops are working with the Iraqi forces ahead of an offensive that will be launched on Isis stronghold, Mosul.

On examining the shrapnel, the US troops found a tar-like black oily substance. The substance tested positive for mustard agent on the first test, but was negative during the second test. The sample has been sent to a lab for conclusive results.

The US official also said that the shell was "militarily ineffective."

Our concern is not much greater after seeing this," he said, according to AFP.

Another US defence officials said the agent had "low purity" and was "poorly weaponised," according to CNN.

However, another officials said: "I don't know of a case like this where it was proximate to US forces like this before." He added that "potentially" the rocket round was "within hundreds of yards" of the US forces and "within the security perimeter" of the base.

The US has destroyed factories and other sites where they believed mustard agent was being produced. Isis has used the mustard agent, aka sulphur mustard that burns the skin, respiratory tract and eyes, earlier as well, killing thousands of people in Isis's other stronghold - Syria.

The US and Iraqi forces are in the last stage of designing an attack on Islamic State militants in Mosul.