Islamic State forces have allegedly fired a shell containing mustard agent onto a military base in northern Iraq, where US and Iraqi troops were present, CNN reports.

According to several US officials, the artillery shell landed on the base south of Mosul, as troops prepared an attack to retake the town from ISIS.

US troops were said to have tested the chemical agent, with the initial reading coming back positive for a mustard agent.

The second test was negative and the substance has been sent for further examination, CNN reported.

No US troops were hurt or displayed any symptoms to being exposed to the chemical, such as skin blistering.

US troops involved in the incident went through decontamination showers as a precaution.

One official said the agent had “low purity” and was “poorly weaponised.”

A second official called it “ineffective,” according to CNN. CNN has previously reported on instances where ISIS had fired rounds with mustard agents in Iraq and Syria.