Militia coalition, known as the Popular Mobilisation Units, had not yet played a heavy role in the fighting.
Shia militias say they have launched an assault to the west of Mosul, opening up a new front in the battle to drive ISIL from the country’s second city and the group’s last major bastion in the country.
The coalition of militias, know as the Popular Mobilisation Units, had not played a heavy part in the fighting but the offensive on Saturday indicates a bigger role than many observers had anticipated.
Al Jazeera’s Jamal Elshayyal, reporting from Iraq, said that though many towns and villages on the road to Mosul had been taken by the Iraqi army and Kurdish Peshmerga forces, the city itself - and ISIL’s position there - remained as formidable as it was before the operation to take the city began 12 days ago.
"The idea behind this ... is that the western part of Mosul has been uncontested so far," El Shayyal said. "And that’s probably the most important frontier because it’s the one that leads to Syria."
The militia forces leading the attack plan to cut off the route between Mosul and Syria and help besiege ISIL-held Mosul from all sides, he said.
Some had hoped the Popular Mobilisation Units would not play a large role in the battle for Mosul, particularly as "Sunni Muslims view them to be just as criminal as ISIL," El Shayyal said.
"That’s why in the beginning [of the offensive] it was stressed by [the government] that the operation would be lead by the Iraqi army and the Kurdish Peshmerga forces," he said.
"Now that they have announced an entire frontier led by them, this will cause a lot of concern, especially as there are reports that they are targeting Sunni civilians."
The Mosul offensive involves tens of thousands of soldiers, federal police, Kurdish fighters, Sunni tribesmen and Shia militias.
Many of the militias - considered to be backed by Iran - were originally formed after the 2003 United States-led invasion to fight US forces as well as Sunni fighters. They were mobilised again, and endorsed by the government, when ISIL swept through northern and central Iraq in 2014, capturing Mosul and other key towns and cities.
Also on Saturday, Iraqi troops approaching the city from the south advanced into Shura, a town to the south of Mosul, after a wave of US-led air raids and artillery shelling against ISIL positions inside the town.
Commanders said most ISIL fighters withdrew earlier this week, allegedly with kidnapped civilians, but that US air raids had disrupted the forced march, allowing some civilians to escape.
Separately, Al Jazeera’s Stefanie Dekker, reporting from near the northeastern front line, said that Kurdish Peshmerga forces had now surrounded the town of Bashiqa.
"They aren’t going in. They’re waiting to [do that] in the next couple of days," Dekker said. "It’s been slow because of the tactics ISIL is using - lots of booby traps, IEDs, and car bombs."
Reporting that the forces were "happy with the progress being made", Dekker added that all fronts - the Iraqi army and counterterrorism units in the south, the Kurdish Peshmerga in the northeast and now the Shia militias in the west - will have to consolidate their positions before moving into Mosul city, which is not expected to happen soon.
"The southern front still has a long way to go," she said.