The battle for Mosul is expected to be the biggest in the 13 years of turmoil unleashed in Iraq by the 2003 US-led invasion which toppled former president Saddam Hussein.

The Shia militias aim to capture villages west of Mosul and reach the town of Tal Afar, about 55 km from the city. Their goal is to cut off any option of retreat by ISIS insurgents into neighboring Syria or any reinforcement for their defense of Mosul.

"We are advancing on four fronts. Each group will advance from a different direction until we reach our target. Our sector is 69 kilometers towards the road linking Mosul to Tal Afar. The main target of the offensive launched by the Hashid al-Shaabi is to cut the supply routes from the Syrian borders to Tal Afar and to Nineveh province. This offensive is meant to cut Daesh's routes," Hashid al-Shaabi (Popular Mobilization) forces commander Abu Turab said.

Iraqi soldiers, security forces and Kurdish peshmerga fighters have been advancing in the last 13 days on the southern, eastern and northeastern fronts around Mosul, which remains home to 1.5 million people.

Iraqi military sources say there had been debate about whether or not to seal off Mosul's western flank. Leaving it open would have offered ISIS a chance to retreat, potentially sparing residents from a devastating, inner-city fight to the finish.

Saturday's announcement by the Shia militias added another force to the coalition of fighters seeking to crush ISIS in Iraq, but will also raise concerns about the role the Popular Mobilization fighters will play. Targeting the ISIS-held town of Tal Afar, close to Turkey and home to a sizeable ethnic Turkmen population with historic and cultural ties to Turkey, will alarm Ankara. But before it was seized by ISIS the town also had a strong Shia presence, and its location on the road west to Syria gives it strategic importance in the war.

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