ISIS forces have fired crude chemical weapons at US troops in Iraq, the Pentagon has confirmed, an alarming disclosure that US officials promptly soften as resulting in no deaths or injuries. The attack came from a powdered mustard agent delivered in a mortar or rocket shell and fired on US forces on Tuesday at the Qayyarah West air base near Mosul. The air base, recaptured from ISIS in July, is a pivotal staging ground for a highly anticipated attack on Mosul, Isis’s capital in Iraq approximately 40 miles (65km) to the north.

Mustard, a banned chemical weapon, is relatively easy to manufacture and has a low incidence of lethality in all but extreme doses, such as the bombardment that former Iraqi dictator Saddam Hussein used on Kurdish civilians and Iranian soldiers in the 1980s and early 1990s. The blistering agent is most dangerous when contrived in a gas form, but a Pentagon spokesman stated on Wednesday evening that tests performed indicated ISIS had delivered the “imprecise and crude” weapon in a powdered state.

The navy captain, Jeff Davis said, “It was mustard agent in a powderized form – the same thing we have seen [Isis] use to little effect many times in the past in both Syria and Iraq. No service members showed signs or symptoms of mustard exposure. This attack has not impacted our mission in any way, nor have we changed our security posture in the area around Qayyarah.” In a February interview, CIA director John Brennan stated that ISIS has used chemical munitions on a “number of instances” in Iraq and Syria, including mustard and chlorine. The jihadist militants are believed to have used both chemical agents, delivered in hollow metal pipes used as mortars or in 122mm Grad rockets, on Kurdish forces in Kirkuk and Makhmour.

Some chemical agents and precursor materials for their manufacture had been stored on Iraqi territory that was formerly under control of ISIS. The US military has insisted since 2014 that there is no evidence that ISIS obtained access to them. Alternatively, some of Syrian dictator Bashar Assad’s substantial chemical stocks have been known to survive a 2013 agreement brokered by Russia and backed by the US for their destruction.

US officials on Wednesday did not indicate any knowledge about the provenance of the Isis mustard powder, nor did they provide any specific details about the shell. It was not known whether Isis manufactured the chemical agent itself or used an antiquated stock from either Syria or Iraq. Although the indirect-fire attack did not harm US forces, it appeared to foretell what US and Iraqi forces are in for when the long-delayed offensive to retake Mosul begins. Hundreds of US troops are operating out of Qayyarah West in anticipation of a major battle, heralded and called off multiple times in the two and a half years since ISIS captured Mosul.

Ashton Carter, the US defense chief, has publicly cited the fall of Mosul and ISIS’ Syrian capital, Raqqa, as a major goal for 2016, before Barack Obama abdicates the presidency. Regarding, potential chemical attack, Pentagon spokesman Davis said: “We are well trained and equipped for this, as are our ISF [Iraqi security force and [Kurdish] Pesh partners.”