Yesterday’s move was part of a bid to encircle the IS-held city and sever supply lines from neighbouring Syria.
Other Iraqi forces, aided by US-led airstrikes and heavy artillery, have driven IS from the town of Shura, south of Mosul, where the militants had rounded up civilians to be used as human shields.
The twin thrusts come nearly two weeks into the offensive to retake Iraq’s second largest city.
However, most of the fighting is still taking place in towns and villages far from its outskirts, and the entire operation is expected to take weeks, if not months.
The involvement of the Iranian-backed Shiite militias has raised concerns that the battle for Mosul, a Sunni-majority city, could aggravate sectarian tensions. Rights groups have accused the militias of abuses against civilians in other Sunni areas retaken from IS. Militia leaders have denied the accusations.
Leaders of the umbrella group for the militias, known as the Popular Mobilization Units, say they will not enter Mosul itself and will instead focus on retaking Tal Afar, a town to the west that had a Shiite majority before it fell to IS in 2014.
Ahmed al-Assadi, a spokesman for the group, said the militias had retaken ten villages since the start of the pre-dawn operation.
However, it is likely there is still some fighting underway. Al-Assadi said forces had been removing explosive booby-traps left by IS to slow their advance.
Jaafar al-Husseini, a spokesman for the Hezbollah Brigades, said his group and the other militias had advanced four miles toward Tal Afar, using anti-tank missiles to destroy three suicide car bombs that were heading towards them.
He said the US-led coalition, which is organising airstrikes and ground support, was not playing any role in the Shiite militias’ advance. However, assistance is being provided by Iranian advisers and Iraqi aircraft.
Many of the militias were originally formed after the 2003 US-led invasion to battle American forces and Sunni insurgents.
They were mobilised again and endorsed by the state when IS swept through northern and central Iraq in 2014.
Iraqi troops approaching Mosul from the south advanced into Shura after a wave of airstrikes and artillery shelling against militant positions inside the town.
Commanders said most of the IS fighters withdrew earlier this week with civilians.
“After all this shelling, I don’t think we will face much resistance,” Iraqi army Major General Najim al-Jabouri said as the advance got underway. “This is easy, because there are no civilians left.”