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One of them is Sheikh Nazhan Sakhar, with 700 men under his command. He says his militiamen are critical to the fight for Iraq's second-largest city because, unlike most of those serving in of Iraq's military, they are local to the Mosul area.

Yet hundreds of the Sunni militiamen have not been paid for months and are viewed with suspicion by the government because many of their relatives have joined Islamic State [IS] group.

The battle for Mosul, which has been under IS control since June 2014, promises to be the biggest and perhaps the last major battle against the militant group in Iraq.

Iraqi forces have already taken back key cities in the sprawling western Anbar province, including Fallujah and the provincial capital of Ramadi.

Fierce fighting is already raging and the focus now is on the push north from the Qayarah air base, with the troops attempting to clear villages south of Mosul.

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Involving local Sunni forces in the fight is critical to not only defeating IS but also keeping the peace after victory, Sakhar says.

"The government pays thousands of soldiers who don't even show up to their posts," he told AP, "Some of my men who were martyred hadn't received a salary in three months."

From the militia's base, the front lines and the Qayarah air base are visible just across the Tigris River.

Sakhar said IS had lost a lot of ground south of Mosul, but could still mount counterattacks.

Earlier this week reports emerged over an IS plan to set the Tigris river on fire in attempt to stop advances towards the city.

The militant group seeks to pump large quantities of oil into the river and set it on fire to hamper the army's plan.

Iraqi forces had constructed a floating bridge on the Tigris south of Mosul to facilitate the transport of troops, vehicles and equipment from its base in Makhmour during the operation to retake the city.

The group's "hysterical" response will create a major obstacle for advancing forces.

Large fleeing civilian population

The Mosul offensive is expected to be far more complicated than other battles, in part due to the large civilian population that has remained in the city – with estimates ranging between 500,000 and one million people that could be trapped by the fighting.

Iraqi commanders say good, local intelligence will be needed to prevent the thousands of IS fighters estimated to still be in Mosul from melting into the civilian population as people flee

The Geneva-based International Committee of the Red Cross said last week that up to one million Iraqis could be forced to flee their homes in the coming weeks.

Iraqi commanders say good, local intelligence will be needed to prevent the thousands of IS fighters estimated to still be in Mosul from melting into the civilian population as people flee.

Iraqi leaders have repeatedly stated that the operation to retake Mosul will commence this year, but the Baghdad government has not yet made any definite announcement or finalised plans for the operation or its expected humanitarian fallout.

Mosul is the last major Iraqi city under IS control after the group lost Fallujah in June, however experts warn the group might resort to a "scorched earth" policy to retain its position in the northern Iraqi city.

For now, Sakhar and his men are glad to pose with their battlefield trophies – such as a black IS flag captured when they took the village of Hajj Ali.

Agencies contributed to this report