Iraqi paramilitary forces have been ordered to retake the town of Tal Afar and prevent Islamic State group jihadists from fleeing west from Mosul towards Syria, a spokesman said Tuesday.

The Hashed al-Shaabi, an umbrella organisation for the paramilitary forces that are dominated by Iran-backed Shiite militias, has played major roles in previous battles against IS, but has been largely on the sidelines since the operation to retake Mosul was announced last week.

The Hashed leadership has ordered "us to assume the mission of liberating the Tal Afar district," Jawwad al-Tulaibawi, spokesman for the Asaib Ahl al-Haq militia, said in a statement.

The Hashed's mission will be to "cut off and prevent the escape of (IS) toward Syria and fully isolate Mosul from Syria," Tulaibawi told AFP.

"We expect that it will be a difficult and fierce battle," he said.

The involvement of the Hashed in the Mosul operation, especially in eventual fighting inside the city itself, has been a source of contention.

Iraqi Kurds and Sunni Arab politicians have opposed its involvement, as has Turkey, which has a military presence east of Mosul despite repeated demands by Baghdad for the forces to be withdrawn.

Relations between the Hashed and the US-led coalition fighting IS are also tense, but the paramilitaries enjoy widespread support among members of Iraq's Shiite majority.

Tal Afar was a Shiite-majority town before the Sunni extremists of IS overran it in 2014, and its recapture is a main goal of Shiite militia forces.

Iraq announced the launch of the operation to retake Mosul on October 17, and have been advancing towards the city from the south, east and north.

But fighting has yet to begin on the western approach to the city, which is exposed to jihadist-held areas between it and Syria.

IS overran large areas north and west of Baghdad in 2014, but Iraqi forces backed by US-led air strikes have since regained significant ground.

Mosul is now the last IS-held city in Iraq, but its recapture will not end the threat of jihadist bombings and other attacks that have plagued the country for years.