Iraq's armed forces will move to retake the major northern city of Mosul from Islamic State once they capture the western city of Ramadi, Prime Minister Haider al-Abadi said on Friday.

Capturing Mosul would deprive the militant group of its biggest population center in both Iraq and Syria, effectively abolishing the state structure of IS in Iraq, depriving it of a major source of funding and dealing a blow to its influence.

The capture of Ramadi would give the army a major psychological boost in its move toward Mosul.

The cities are about 420 km (260 miles) apart by road.

Iraqi forces started an attack on Tuesday to dislodge Islamic State militants from the center of Ramadi, the last district under their control in the city they had captured in May. Islamic State seized Mosul, Iraq's second-largest city, in 2014, in an offensive that forced government forces to abandon a third of Iraqi territory.

The group declared a caliphate over the Iraqi and adjacent Syrian Sunni populated territory it controls.

"The liberation of dear Mosul will be achieved with the cooperation and unity of all Iraqis after the victory in Ramadi," Abadi said in a statement on the state media website on Friday.

Army commanders said on Wednesday that the battle for Ramadi would take several days.