Iraqi forces backed by paramilitary units were preparing Monday to attack the last stronghold of the Islamic State group in the western desert bordering Syria, an AFP correspondent said.

Armoured vehicles advanced on a road across a rocky plain as troops on pick-up trucks armed with machineguns watched from behind dusty embankments.

The vehicles were approaching Anna, one of three towns still in the jihadists' hands about 100 kilometres (60 miles) from the Syrian border, in the vast province of Anbar.

After retaking Anna and then Rawa, the Iraqi forces will target Al-Qaim, the last of the towns before the border with the Syrian province of Deir Ezzor.

The IS jihadists are also under pressure from Syrian government forces backed by Russian air power and an alliance of Arabs and Kurds supported by a US-led coalition.

"The objective is to bring the entire province of Anbar back into the fold of the nation," Lieutenant General Rashid Flaih, head of Anbar's paramilitary units, told AFP.

Operations had already been carried out and "many jihadists have been killed," said General Abed Jabbur Mathlum, the deputy commander for the area.

The battle for the towns, where more than 1,500 jihadists are thought to be holed up, comes as Iraq prepares to launch an assault on Hawija, another IS stronghold about 300 kilometres north of Baghdad.

Iraq dealt IS a heavy blow in July when it recaptured second Mosul from the jihadists three years after they overran swathes of the country's north.