Isil militants are expected to use chemical weapons in a desperate defence of Mosul as Iraqi troops prepare to retake the country’s second city, a senior coalition officer has warned.

The recapture of Mosul, which has been in the hands of Islamic State in Iraq and Levant (Isil) for nearly two years, is “inevitable”, but will be a “long, hard road,” said Brig Roger Noble.

The city is believed to have been fortified with tunnel complexes and minefields made up of thousands of homemade bombs. Iraqi commanders backed by US-led air strikes are also likely to face a familiar barrage of suicide bombers both on foot and in vehicles.

Brig Noble, the Australian deputy commander of international troops training and supporting the Iraqi army, said generals were expecting a repeat of recent chemical weapons attacks seen on Kurdish troops.

He said: “Chemical weapons are part of the equation and the Iraqi army plans for and operates with that as part of the equation. They would expect the full raft of enemy capabilities to be employed.”

Brig Noble, who keeps his own gas mask next to his desk, said both the Iraqis and US-led coalition are “acutely on guard” for chemical attacks.