Iraq's prime minister says government forces have begun the operation to retake the town of Hawija from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

In a statement released by his office, Haider al-Abadi said the operation began at dawn on Thursday, just two days after Iraqi forces began an offensive against ISIL holdouts in the western Anbar province.

"We announce the launch of the first phase of the liberation of Hawija, in fulfilment of our pledge to our people to liberate all Iraqi territory and to cleanse it from the terrorist Daesh gangs," Abadi said, using the Arabic acronym for ISIL.

Abadi praised his forces, who were "fighting more than one liberation battle at the same time and winning victory after victory", adding that "a new victory" was looming.

Al Jazeera's Hoda Abdel-Hamid, speaking from Sulaimaniyah city in Iraq's Kurdish region, said that up to 2,000 ISIL fighters were estimated to be in Hawija.

"Some have argued that Hawija should have been dealt with before the Mosul offensive as there's a sizeable number of ISIL fighters - estimated between a 1,500 to 2,000. And that the Hawija district has actually acted as a rear base for the ISIL fighters, somewhere where they could have retreated safely," she said.

The offensive started from the northwest and the southwest of Hawija district, she said.

"It is going to take a while because the area is full of small villages and it's quite vast and porous. So they will have to try to encircle Hawija town before they are able to get into it."

One of the last

Hawija, located in the oil-rich province of Kirkuk, 240km north of Baghdad, is one of the last pockets of territory held by ISIL fighters in Iraq.

Iraqi and coalition planes stepped up a campaign of air raids on Hawija earlier this month, targeting ISIL bases and weapons facilities.

Iraqi forces have already recaptured the northern cities of Mosul and Tal Afar this year.