Iraqi Shi'ite militias backed by Iran will soon join the fight against Islamic State on a new front west of Mosul, a move which could block any retreat by the jihadists into Syria but might alarm Turkey and the United States.

The Shi'ite militias, with thousands of battle-hardened fighters trained by Iran, would bring important extra firepower to what is expected to be the biggest battle in Iraq since the US-led invasion in 2003.

But their arrival on the battlefield in one of the most diverse parts of Iraq also creates worry for Western countries backing the Iraqi government offensive, who fear that the Shi'ite fighters could alienate residents in mainly Sunni areas.

A spokesman for the paramilitary groups said the advance towards the IS-held town of Tal Afar, about 55 km west of Mosul, would start within "a few days or hours".

If successful, the offensive would leave IS fighters - and the 1.5 million civilians still living in Mosul - encircled by an advancing coalition of forces which seeks to crush the hardline Sunni militants in their Iraq stronghold.

As many as 50,000 Iraqi soldiers, police and Kurdish peshmerga fighters, backed by US-led air strikes and support on the ground, have advanced on Mosul for nearly two weeks from the south, north and east.

Rights groups have called on Baghdad to keep the Shi'ite militias away from the battlefield, accusing them of carrying out revenge killings and kidnappings in other areas freed from IS. The militias and the Baghdad government say any such abuses were isolated incidents and not widespread.

The battle for Mosul itself, a city many times larger than any other ever held by IS, is expected to be the biggest military operation in Iraq since US troops invaded to topple former president Saddam Hussein nearly 14 years ago.

Adding to the challenges facing the advancing forces, retreating IS fighters have forced women and children from outlying villages to march alongside them as human shields as they withdraw into the city, according to villagers who spoke to Reuters by telephone from Mosul.

Older boys and men of fighting age were taken off to an unknown fate, they said.

The United Nations said on Friday IS had abducted 8000 families from around Mosul to use as human shields, and had killed 232 people near the city on Wednesday who refused to comply with orders.

The group's "depraved, cowardly strategy is to attempt to use the presence of civilians to render certain points, areas or military forces immune from military operations, effectively using tens of thousands of women, men and children as human shields," UN High Commissioner for Human Rights Zeid Ra'ad al Hussein said.

Ahmed al-Asadi, a spokesman for the Shi'ite forces known collectively as the Hashid Shaabi, or Popular Mobilisation, said the operation to cut off Mosul's western approaches was crucial to defeat IS, also known as Daesh, ISIS or ISIL.