Iraqi forces announced the launch of a major operation on Thursday to flush remaining Islamic State group fighters out of the western desert near the border with Syria.

The arid, sparsely populated wastelands between the Tigris and Euphrates rivers are the last refuge of the jihadists in Iraq after troops and paramilitaries ousted them from both valleys and all urban areas.

"The Iraqi army, the federal police and the Hashed al-Shaabi (Popular Mobilisation paramilitary units) this morning began clearing the Al-Jazeera region straddling Salaheddin, Nineveh and Anbar provinces," the head of Joint Operations Command, General Abdelamir Yarallah, said in a statement.

The region's dry valleys, the oases and steppes make up around four percent of national territory, Hisham al-Hashemi, an Iraqi expert on IS, told AFP last week.

Iraq's close ally Iran has already declared victory of IS but Prime Minister Haider al-Abadi said on Tuesday that he would not follow suit until the desert has been cleared.

"After the operation has ended, we will announce the final defeat of Daesh in Iraq," he said, using an Arabic acronym for IS.

Over the border in neighbouring Syria, pro-government forces and US-backed Kurdish-led forces are conducting similar operations to clear IS fighters from the countryside north of the Euphrates valley after ousting them from all urban areas.