Amnesty International "extrajudicial executions" of Sunni detainees took place early this month by Iraqi government forces and Shia militias in the Iraqi cities of Tal 'Afar, Mosul and Ba'quba.
In a statement issued by the international human rights organisation, these "vengeful" acts were described as "war crimes."
The organisation said: "Surviving detainees and relatives of those killed gave graphic accounts that suggest Iraqi forces had carried out a series of vengeful attacks against Sunni detainees before withdrawing from Tal 'Afar and Mosul in northern Iraq."
Ba'quba mayor Abdallah Al-Hayali told the organisation that his 21 year-old nephew was among up to 50 people "extrajudicially executed" in an incident at Al-Wahda police station in the Mufaraq district of central Ba'quba in the early hours of 16 June.
Amnesty International continued: "According to a medical report obtained by the mayor, he had been shot in the head. Many of those killed with him had been shot in the head and the chest. Sunni policemen who witnessed the killing fled their posts afterwards for fear of reprisal."
Amnesty International's senior crisis response adviser Donatella Rovera, who is currently in northern Iraq, said: "Reports of multiple incidents where Sunni detainees have been killed in cold blood while in the custody of Iraqi forces are deeply alarming."
She continued: "The killings suggest a worrying pattern of reprisal attacks against Sunnis in retaliation for ISIS gains."