The head of the Defense Intelligence Agency says the urban battle to retake Iraq's second largest city from Islamic State militants that he predicts will unfold in the next two or three months will be "long and difficult."

Lt. Gen. Vincent Stewart said at a security forum on Thursday that urban warfighting in Mosul will not be easy and noted the militants have had at least two years to prepare to defend their position.

He also warned of the danger in retaking Mosul before governance and humanitarian resources are in place.

Late last month, Iraqi forces retook a town 45 miles south of Mosul. A string of towns south and southeast of Mosul have also been recaptured as part of an operation aimed at eventually unseating IS from Mosul itself.