French Defence Minister Jean-Yves Le Drian said Tuesday that Iraqi forces supported by international strikes are working to encircle Islamic State group stronghold Mosul in preparation for the battle to retake it.

"We are in the process of surrounding Mosul to prepare for the battle, which will be tough," Le Drian told journalists in Arbil, the capital of Iraq's autonomous Kurdish region.

Le Drian, on the second day of an unannounced trip to Iraq, said warplanes from a US-led coalition, including four from France, had recently struck IS command centres in Mosul, located in the country's north.

On Monday, Le Drian said that Mosul and Syria's Raqa, another key IS-held city, "must fall" in 2016.

Le Drian's remarks were the most specific timetable for the cities' recapture given by a member of the US-led coalition against the jihadists, which has been reluctant to comment on the expected pace of operations.

The coalition is carrying out strikes against IS and providing training and other assistance to forces fighting the jihadists.

Raqa was seized by IS in early 2014, and Mosul was overrun during a jihadist offensive in June that year.

The fact that both cities still have large civilian populations will complicate efforts to retake them, and the jihadists have had ample time to sow slews of bombs and set up other defences.

IS claimed attacks in Paris that killed 130 people in November last year, and there is concern that the jihadists will strike the country again.

Belgium's federal prosecutor has said a jihadist cell that attacked Brussels airport and a metro station last month, killing 32 people, initially planned to target France.