“You don’t liberate a heavily defended city the size of Mosul quickly, you just don’t. If an enemy wants to hold a city the size of Mosul, it will take time to clear it,” he said.

Iraqis soldiers were advancing “in face of an enemy who is being a barbaric as he’s ever been,” said Maj Gen Jones.

He said: “He is being relentlessly evil in the streets of Mosul. Beheadings, throwing people in oil pits, it’s just repugnant to any civilised society.”

Maj Gen Jones said patience was needed to liberate Mosul, but said he hoped the fighting would be finished sometime in the spring.

The upcoming struggle for Raqqa will be worse, he predicted. Syrian rebels poised to start encircling the centre of the caliphate lack much of the equipment the Iraqi Army has.

Donald Trump has appeared to criticise coalition commanders for not doing enough against Isil and has promised to “bomb the hell” out of the militants.

But Maj Gen Jones said Isil has lost between 50 and 60 per cent of its territory in Iraq and were being hit “very, very hard”.

He said: “By the time the Iraqi forces stepped off into Mosul, we had been shaping a degrading that enemy day in, day out for months. Go and ask your Daesh commander whether he’s been hit hard enough.”