Iraqi forces said Thursday they had retaken the centre of Islamic State group stronghold Hawija and were pushing forward in their assault on one of the last IS enclaves in the country.

Troops, police and paramilitaries "liberated the whole of the centre of Hawija and are continuing their advance," the operation's commander, Lieutenant General Abdel Amir Yarallah, said.

Government and allied forces backed by a US-led coalition launched an offensive last month to oust IS from Hawija, a longtime insurgent bastion.

The town is among the final holdouts from the territory seized by the jihadists in 2014 and its recapture would leave only a handful of remote outposts in IS hands.

Iraqi Prime Minister Haider al-Abadi was due to meet ​French President Emmanuel Macron in Paris later on Thursday for talks on the campaign against IS, which is backed by a US-led coalition including France.

The United Nations said on Tuesday that an estimated 12,500 people had fled Hawija since the launch of the offensive to retake the town and surrounding areas last month.

The UN's humanitarian affairs office said the number of people still in the town was unknown but could be as high as 78,000.

It said humanitarian agencies have set up checkpoints, camps and emergency sites capable of receiving more than 70,000 people who could flee.

- Longtime insurgent bastion -

Hawija, 230 kilometres (140 miles) north of Baghdad, is one of just two significant areas of Iraq still held by IS, along with a stretch of the Euphrates Valley near the Syrian border which is also under attack.

It has been an insurgent bastion since soon after the US-led invasion of 2003, earning it the nickname of "Kandahar in Iraq" for the ferocious resistance it put up similar to that in the Taliban militia's citadel in Afghanistan.

The town's mainly Sunni Arab population is deeply hostile both to the Shiite-led government in Baghdad and to the Kurds who form the historic majority in adjacent areas.

Hawija lies between the two main routes north from Baghdad -- to second city Mosul, recaptured from IS in July, and to the city of Kirkuk and the autonomous Kurdish region.

Last year, government forces bypassed the area in their advance north to Mosul, which culminated in the jihadists' defeat in their most emblematic bastion.

IS has been forced out of most of the territory it seized in Iraq and Syria during a lightning offensive in the summer of 2014 that was followed by its declaration of a cross-border "caliphate".

The US-led coalition is also backing an Arab-Kurdish alliance, the Syrian Democratic Forces, that is battling to oust IS from its de facto Syrian capital Raqa.

The SDF has captured about 90 percent of Raqa and is fighting fierce battles with remaining IS jihadists.

IS's other main stronghold in Syria is the eastern province of Deir Ezzor, which borders IS-held territory in Iraq.

Two separate offensives are under way against the jihadists in Deir Ezzor -- one by the SDF, the other by government forces supported by Russia.