Unidentified assailants on Wednesday night attacked a convoy of Hashed al-Shaabi fighters who were going on leave and therefore unarmed, the security forces' media centre said.

"Six fighters were killed and 31 more were wounded and transferred to the Qayyarah hospital," the statement said.

The attack took place on the road between Mosul and the town of Makhmur, areas that Iraqi troops and allied paramilitary units recaptured from the Islamic State group in 2017.

There was no immediate claim of responsibility for the killings.

"It's the first attack of its kind after the liberation," Rezkar Mohammad, a local official in Makhmur, told AFP.

"The area is vast and open, and we already warned forces here that it hadn't been totally cleared," he said.

Iraqi forces had deployed in the area in pursuit of the attackers, Mohammad said.

Baghdad declared victory against IS in late 2017 but jihadist sleeper cells still mount hit-and-run attacks against military positions, particularly in Iraq's north and west.

Security forces have also warned that IS fighters could slip across the porous border from neighbouring Syria to escape a blistering US-backed assault on their final redoubt there.