Iraqi Kurdish leader Massud Barzani said Saturday that preparations for the operation to retake Mosul from jihadists have been completed and it is time for the battle to begin.

Forces from Iraq's autonomous Kurdish region are expected to play a major role in the battle for Mosul, the country's second city, which has been held by the Islamic State group since June 2014.

"The time has come to begin the liberation of Mosul," a post on Barzani's official Twitter account said.

"All preparations for the battle to liberate Mosul have been completed," he said in a statement on the Kurdish regional presidency's website.

The launch of the operation is expected to be announced soon, but it will mark only the start of a battle that is likely to be the most difficult and complex yet in the war against IS.

Once the battle is announced, a coalition of heterogenous and sometimes rival Iraqi forces will have to fight their way through IS defences to reach the city, in some cases over distances of dozens of kilometres (miles).

Then they will likely seek to surround the city before launching an assault, marking the start of deadly street fighting with die-hard jihadists in a city with a large civilian population.

The battle may spark a humanitarian crisis, with the United Nations warning that up to one million people may be displaced by the fighting as winter sets in.