Among the Iraqi forces preparing for the key battle to retake the Islamic State-held city of Mosul are Sunni tribal militias, drawn from the local villages and motivated by the desire to claw back home ground lost to the militants over two years ago.

One of them is Sheikh Nazhan Sakhar, with 700 men under his command.

Sakhar 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 haven't been paid for months and are viewed with suspicion by Iraq's government because many of their relatives have joined the IS militant 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 Islamic State 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 airbase, with the troops attempting to clear villages south of Mosul. 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 said. "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 airbase are visible just across the Tigris River. Sakhar said IS had lost a lot of ground south of Mosul, but could still mount counterattacks.

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 1 million people that could be trapped by the fighting.

The Geneva-based International Committee of the Red Cross said last week that up to 1 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 finalized plans for the operation or its expected humanitarian fallout.

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

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Associated Press writer Salar Salim contributed to this report.