Iraqi militias are recruiting children from camps for civilians displaced by conflict ahead of the long-awaited operation to retake militant-held Mosul, according to a report from Human Rights Watch Tuesday.

Citing testimony from witnesses and relatives, HRW said two tribal militias in the Kurdish region of Iraq recruited children from a camp south of Irbil and drove them away to a town near Mosul.

The group said the recruits are intended to reinforce frontline positions against the Islamic State group in Nineveh province, where Mosul is located.

IS uses children as both frontline fighters and suicide bombers in Iraq and Syria. The group maintains an army of child soldiers, which it calls "cubs of the caliphate." Islamic State videos have shown boys killing IS opponents through beheadings and shootings.

"The (Iraqi) government and its foreign allies need to take action now, or children are going to be fighting on both sides in Mosul," Bill Van Esveld of Human Rights Watch stated in the group's report.

Iraq's prime minister has pledged that Mosul will be retaken from IS this year. After a string of territorial defeats over the past year, Mosul, Iraq's second largest city, is the last major urban territory IS holds in the country.