Pro-Baghdad militias, incorporated into the Iraqi as popular mobilisation forces are recruiting children to fight in their ranks from internal displacement camps as they prepare for an assault on the Isis-held stronghold of Mosul.

The international NGO Human Rights Watch has documented the recruitment of child soldiers into the Hashad al-Asha'r, a Sunni militia which is expected to engage against the Islamic State in Mosul, alongside the regular Iraqi Security Forces.

The children were taken from Debaga refugee camp, 40km south of Erbil, which currently houses over 35,000 people displaced in the fighting between government forces and Isis (Daesh).

Two people living in the camp since March told Human Rights Watch that at least two militia groups engaged in the fighting against Isis are entirely made up of camp residents.

The militias, which are funded directly from Baghdad, recruited at least seven children on August 14 when they took away about 250 new recruits.

An aid worker who had been monitoring developments on the ground said recruits from camps were being transferred to front lines. He added he had seen men wearing ISF uniforms in Debaga camp a few days before the August 14 transfer and that near the front lines, militia members fought in Security Force uniforms.

Human Rights watch has called in all parties in the Iraqi conflict to demobilise child fighters under the age of 18.

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

The use of child soldiers by the Islamic State has been well documented. The militant group has used fighters under the age of 16 extensively in their propaganda campaigns.

Reports have emerged from Mosul, Iraq's second largest city, that in the build up to the Iraqi offensive on the city the Islamic State is also increasingly relying on child soldiers as its other fighters desert from its ranks.