British Defence Minister Michael Fallon said Saturday in Iraq that he expected to see the Islamic State group expelled from the country's major towns by the end of 2017.

"We expect to see Daesh (IS) expelled from the major towns and cities of Iraq during the course of the year," he told reporters in Arbil, the capital of Iraq's northern autonomous region of Kurdistan.

Iraqi forces are nearly four months into a massive operation to retake nearby Mosul, which is the country's second city and where IS supremo Abu Bakr al-Baghdadi proclaimed a "caliphate" in 2014.

The jihadist organisation then controlled around a third of Iraq, but federal and allied forces have since retaken around two thirds of that territory and Mosul is IS's last major stronghold.

After retaking the eastern side of Mosul last month, Iraqi forces are currently preparing to launch an assault on the part of the city that lies west of the Tigris River.

Commanders expect the battle to be fierce because the narrow streets of the Old City will complicate operations and the western side also harbours some traditional jihadist bastions.

Iraqi Prime Minister Haider al-Abadi said around the new year that he expected his forces would need three more months to rid the country of IS.

Most observers argued that the premier's prediction was optimistic, however, with Mosul alone threatening to bog down Iraqi forces way past that target.

Retaking the northern city would deal a death blow to the "caliphate" and any claim that IS is still running a "state", but the group retains control of several populated areas.

In Iraq, IS still holds Hawijah, a large town southeast of Mosul, and the town of Al-Qaim on the western border with Syria.

When Iraqi forces retake Mosul, the jihadists' last major hub will be the city of Raqa in neighbouring Syria.

"The situation in Syria is more complicated, given the continuation of the civil war there," Fallon said.

A 60-nation coalition led by the United States has carried out thousands of air strikes in support of the war on IS and provided assistance and training to thousands of Iraqi forces.

Britain is a key member of that coalition, together with France, Italy and Australia.

Fallon said the Royal Air Force had struck 300 targets in and around Mosul since the operation to retake the city began on October 17.