ERBIL, Iraq (Reuters) – Iraq’s Shi’ite paramilitaries launched an offensive on Friday to drive Islamic State from a desert region near the border with Syria as security forces fought the militants in the city of Mosul.

Spokesman Karim al-Nouri said the target of the operation was the Qairawan and Baaj areas about 100 km west of Mosul, where U.S.-backed Iraqi forces are advancing in their campaign to rout the militants from city.

Seven months into the Mosul campaign, Islamic State has been driven from all but a handful of districts in the city’s western half including the Old City, where it is using hundreds of thousands of civilians as human shields.

The paramilitaries have been kept on the sidelines of the battle for the city of Mosul itself, but have captured a vast, thinly populated area to the southwest, cutting Islamic State supply routes to Syria.

Islamic State is losing territory and on the retreat in both Iraq and Syria.

The Iraqi military said in a statement its air force was supporting the operation by the paramilitary groups known collectively as Hashid Shaabi or Popular Mobilisation Forces (PMF).

Unlike regular Iraqi security forces, the PMF does not receive support from the U.S.-led coalition, which is wary of Iran’s influence over the most powerful factions within the body.

Officially answerable to the government in Baghdad, the PMF were formed when Islamic State overran around one third of Iraq including Mosul nearly three years ago and Iraqi security forces disintegrated.

Nouri said PMF control over the border would assist Syrian government forces when they push toward the Islamic State-held city of Raqqa.

On Friday, the Syrian Democratic Forces (SDF) said their assault on Raqqa, the militants’ biggest urban stronghold, would begin soon and that they were awaiting weapons including armored vehicles from the U.S.-led coalition

The PMF is not officially involved in Syria, but tens of thousands of Iraqi Shi’ite militiamen are fighting there on behalf of the government of President Bashar al-Assad, which is backed by Iran.

(Reporting by Isabel Coles; Editing by Richard Lough)