At least 20 Sunni tribal fighters were killed in an airstrike south of Mosul in the early hours of Wednesday when they were mistaken for Islamic State militants, Iraqi police said.
Whether the aircraft that carried out the attack belonged to the Iraqi air force or the US-led coalition could not be determined.
A spokesman for the coalition in Baghdad couldn't confirm if it had planes in the air at the time of the attack, around 1 a.m. (2200 GMT Tuesday), near Qayyara, according to a local police and a tribal leader.
Qayyara is located 60 kilometers (about 40 miles) south of Mosul, Islamic State's stronghold in northern Iraq.
Iraqi forces dislodged the militants from the town in August with support from the coalition, which plans to use a nearby airfield as hub to support the offensive on Mosul that could start this month, according to local commanders.