HRW said it documented cases of screening and torture of villagers by Hashd al-Shaabi units during the battle to take the city of Hawija
Human Rights Watch on Thursday condemned abuse by paramilitary units fighting alongside Iraqi security forces to expel Islamic State group fighters from their last redoubts in the country.
"Right now, the Iraqi government’s attitude seems to be 'all hands on deck' for these last battles against ISIS (IS)," said HRW's Middle East director, Sarah Leah Whitson.
"While Iraqi forces do need all the help they can get, the government should not allow abusive forces to use this opportunity for even more abuse."
Despite pledges to the contrary, Prime Minister Haider al-Abadi was allowing the Popular Mobilisation units (PMU) also known as Hashd al-Shaabi "to play a more prominent role not only in the fighting but also in screening and detaining people during military operations," the New York-based rights watchdog said.
"Iraqi military forces are taking the law into their own hands, playing judge, jury and executioner with captive ISIS suspects," Whitson said.
HRW said it had documented cases of screening and torture of suspects from interviews with villagers displaced by the latest fighting to retake the northern city of Hawija.
Families had given accounts of how Hashd units had detained and beaten male villagers and taken away four men from Sayhat Othman, 42 kilometres northwest of Hawija, who had not been seen again, sparking rumours they had been shot dead.
One of the men, who had surrendered to Iraqi forces, was an amputee in a wheelchair, who had worked in the local mosque under IS "and cooperated with the group while they were in control of the area", HRW quoted family members as saying.
After the defeat of IS in Iraq's second city Mosul in July and the recapture of adjacent areas, the Hawija area represents the last enclave still held by the militants in Iraq apart from a section of the Euphrates Valley near the border with Syria.