Iraq brought all of its territory still held by the Islamic State group under attack Thursday, announcing an assault on the second of two remaining jihadist enclaves.

Prime Minister Haider al-Abadi said the attack on the besieged IS pocket around the mainly Sunni Arab northern town of Hawija, began at dawn and predicted it would bring a new victory against the crumbling jihadists. 

The enclave, which was bypassed by government forces in their drive north to second city Mosul last year, has been a bastion of insurgency ever since the first year of the US-led occupation in 2003.

After the defeat of IS in Mosul in July and the recapture of adjacent areas, Hawija and neighbouring towns form the last enclave still held by IS apart from a section of the Euphrates Valley downstream from the Iraqi border.

"At the dawn of a new day, we announce the launch of the first stage of the liberation of Hawija, in accordance with our commitment to our people to liberate all Iraqi territory and eradicate Daesh's terrorist groups," Abadi said, using an Arabic acronym for IS.

"Greetings to all of our forces, who are waging several battles of liberation at the same time and who are winning victory after victory and this will be another, with the help of God," he said.

An AFP correspondent heard heavy shelling around the IS-held town of Sharqat where Iraqi forces have been massing in recent days.

- 'Kandahar of Iraq' -

Hawija earned the nickname of "Kandahar in Iraq" from US-led coalition troops from the early months after the invasion of 2003 for the ferocious resistance it put up similar to that in the Taliban militia's bastion in Afghanistan.

Located west of the ethnically divided Kurdish-held city of Kirkuk, Hawija also lies on a fault line of Arab-Kurdish tensions.

Despite forming part of Kirkuk province, the area is overwhelmingly Sunni Arab and bitterly opposed to Kurdish ambitions to incorporate Kirkuk in their autonomous region in the north.

Preparations for the offensive in Hawija have been overshadowed by an independence referendum that Kurdish leaders  plan to hold on Monday in areas including Kirkuk against the wishes of the federal government in Baghdad.

It is the latest in a string of setbacks for IS in Syria and neighbouring Iraq.

After seizing swathes of Syria and neighbouring Iraq in 2014, IS has seen the territory under its control fast diminish in recent months.

On Tuesday, Iraqi forces launched an attack up the Euphrates Valley against the other one of IS's two remaining enclaves in Iraq.

And in Syria's eastern province of Deir Ezzor, IS faces twin assaults -- one by Russian-backed government troops and the other by US-backed fighters of the Syrian Democratic Forces.

Further up the Euphrates, the SDF now controls 90 percent of the city of Raqa, the Britain-based Syrian Observatory for Human Rights war monitor said Wednesday.

The jihadists seized Raqa in early 2014, making it their de facto Syria capital and a byword for the group's most gruesome atrocities, including public beheadings.

IS is also thought to have used the city to plan attacks abroad.