Shi'ite militias launch attack near Mosul

Shi'ite militias have launched an operation to retake the Iraqi town of Tel Afar from IS but say they won't enter the Sunni-majority city of Mosul near by.

Islamic State had abducted 8000 families from around Mosul to use as human shields. (AAP)

State-sanctioned Shi'Ite militias have launched an assault on the Islamic State group west of the Iraqi city of Mosul but reiterated that they would not enter the Sunni majority city.

Jaafar al-Husseini, a spokesman for the Hezbollah Brigades, said they launched an offensive on Saturday along with other large militias towards the town of Tel Afar, which had a Shi'Ite majority before it fell to IS in 2014.

Iranian forces are advising the fighters and Iraqi aircraft are providing air strikes, he said.

Iraq launched a massive operation to retake militant-held Mosul, its second largest city, last week. The involvement of the Shi'Ite militias has raised concerns the battle could aggravate sectarian divisions.

The Mosul offensive involves more than 25,000 soldiers, federal police, Kurdish fighters, Sunni tribesmen and the Shi'Ite militias, which operate under an umbrella organisation known as the Popular Mobilisation Units.

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, a Sunni extremist group, swept through northern and central Iraq in 2014, capturing Mosul and other towns and cities.

A US-led coalition has been providing air strikes and ground support to Iraqi forces in the Mosul offensive, but al-Husseini said it had no involvement in the Iran-backed militias' advance on Tel Afar.

He said the militias will focus on Tel Afar and on securing the western border with Syria. IS still controls territory on both sides of the border, where it shuttles fighters, weapons and supplies between Mosul and the Syrian city of Raqqa, the de facto capital of its self-styled caliphate.

Iraqi forces advancing toward Mosul from several directions have made uneven progress since the offensive began. Iraqi forces are 6 kilometres from the edge of Mosul on the eastern front, where the elite special forces are leading the charge. But progress has been slower in the south, with Iraqi forces still 35km from the city.

There have been no major advances over the past two days, as Iraqi forces have sought to consolidate their gains by clearing explosive booby-traps left by the extremists and uncovering tunnels they dug to elude air strikes.