Child refugees are being recruited from camps in Iraq to fight ISIS, according to Human Right Watch (HRW).

Government backed tribal militias have taken children from at least one displaced persons camp in the Kurdistan region of Iraq to fight the jihadist group near Mosul, witnesses told the charity. The youngsters were taken from the Debaga camp on August 14 and are feared to be playing a part in the upcoming offensive against the ISIS-held town.

Residents of the camp told HRW that at least two militia groups fighting against ISIS are made up entirely of refugees and have been recruiting from the camp for months. The witnesses said that on August 14 two large vans arrived and took away 250 new fighters, at least seven of whom were under 18.

The refugees were taken to join Hashad al-Asha'ri, a local Sunni militia group which is expected to play a key part in the battle for Mosul as the government may order Shia militias to stay away for fear of sectarian infighting.

One aid worker working in the area said removing people from the camp to the frontline was part of the militia's strategy to fight ISIS.

"The recruitment of children as fighters for the Mosul operation should be a warning sign for the Iraqi government," said Bill Van Esveld, senior children's rights researcher at HRW. "The government and its foreign allies need to take action now, or children are going to be fighting on both sides in Mosul."

He added the US should urge the Iraqi government to ensure troops supporting them don't have under 18s in their ranks.

"The battle for Mosul should not be fought with children on the front lines," he said.