The Sunni fighters, who are aligned with the Iraqi government, are accused of carrying out a campaign of vendetta attacks against villagers they say supported the Islamic State of Iraq and the Levant (Isil) but also using the chaos of the Mosul offensive to settle old scores.
The violence has led to calls for the Iraqi government to “rein in” its militia allies and raised fears about more sectarian bloodshed during the battle to recapture Mosul itself.
Residents of al-Makuk, a village on the the Tigres river, said fighters from the Sab’awi tribe began rounding up men and boys after they entered the town in October.
Other men suspected of supporting Isil were beaten and tied to the front of cars before being driven through the streets. “They called for everyone to come out and see the Daeshi,” said one woman, using the Arabic term for an Isil supporter.
A man named only as “Ahmed” was reportedly dragged out of his home and beaten in front of his wife and children because his brother was accused of supporting Isil.
“They kicked him to the ground and ‘tasered’ him three times. They punched him and beat him with the back of their Kalashnikovs, with metal rods, and even a rubber hose - the thick ones used for agriculture,” a witness said.
“The Iraqi authorities have repeatedly failed to stop revenge attacks or investigate crimes by militias from the Popular Mobilisation Units, who are also participating in the Mosul offensive,” said Lynn Maalouf, research deputy director at Amnesty.
“This has fostered a dangerous culture of impunity in which perpetrators of such attacks feel they have free rein to commit crimes and go unpunished.”
Much of the concern on renegade militia violence has been focused on the Shia militias who fight alongside the government but have carried out bloody reprisals against Sunni civilians in the past.
Shia fighters are not taking part in the attack on Mosul itself but concerns are growing about their plans to attack the town of Tal Afar, to the west of the city.
Tal Afar is majority Shia but US officials are concerned that the militias could persecute the Sunni minority, some of whom cooperated with Isil when the jihadists took the town in 2014.
Meanwhile, Iraqi commandos clashed with Isil fighters in tunnels dug underneath a newly liberated neighbourhood of Mosul as troops went house-by-house to flush out hidden jihadists and improvised explosives.
Special forces units paused their advance into Mosul’s eastern district of Gogjali and commanders said they were focusing on driving out Isil fighters who had hidden in the area and disarming booby traps.
Eight Isil militants were killed in firefights in Gogjali throughout the day. Six were hiding inside a tunnel while two more were shot while charging at Iraqi troops in what appeared to be an attempted suicide bombing, Lt Col Muhanad al-Timimi said. Three others were arrested.
Iraqi troops imposed a curfew on the neighbourhood as engineers searched streets and buildings for hidden bombs. Isil left hundreds of booby traps behind when its forces fled from the city of Ramadi and Iraqi troops fear they may use similar tactics as they are pushed back in Mosul.
The overall advance into Iraq’s second largest city was slowed on Wednesday because of bad weather. Intense humidity and thick clouds made it more difficult for US and Iraqi aircraft to support the advancing troops.