An air strike has killed at least 20 Iraqi pro-government Sunni tribal fighters in an area south of Mosul, according to the security forces and a minister.
It was unclear whether it was the Iraqi military or the US-led coalition against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) that mistakenly hit the fighters on Wednesday.
Mosul is the last major ISIL stronghold in Iraq.
The raid happened at around 1am on Wednesday (22:00 GMT on Tuesday) in the east of the town of Qayyarah, which was recaptured from ISIL in August, the officials said.
Sheikh Nazhan Sakhr al-Lihaybi, the commander of the fighters who were killed, said they had succeeded in repelling an attack by ISIL in the area, and were bombed when they gathered after the end of the fighting.
Agriculture minister Falah Hassan Zeidan, whose tribe resides in the Qayyarah area, also said that 21 tribal fighters were killed and confirmed the timing of the strike, according to the AFP news agency.
The Reuters news agency said 20 fighters were killed in the strike, quoting police sources.
The US-led coalition has been carrying out strikes against ISIL in Iraq since 2014, and the country's military also targets the group with warplanes and helicopters.
The deadly strike comes as an alliance of Iraqi forces prepare for a final push to retake Mosul from ISIL.
The Mosul operation, which Western officials have indicated could start this month, will involve a coalition of sometimes rival Iraqi forces, including soldiers, police, Kurdish fighters, and both Sunni and Shia tribal fighters.