Iraqi forces have illegally executed at least 255 Sunni prisoners in the past month, according to a report by Human Rights Watch (HRW).
The group said it found evidence of summary killings by the Iraqi army and militias affiliated with the government since 9 June.
In nearly all cases, soldiers, police or Shia militia members shot the prisoners dead, but in one case dozens of prisoners were reportedly set on fire and in another, grenades were thrown into locked cells.
Five massacres were documented in Mosul and Tal Afar, in northern Nineveh province, in Baaquba and Jumarkhe, in Diyala, and in Rawa in Anbar province.
The killings appeared to be retaliation for attacks by the Islamic State of Iraq and the Levant (Isis), which has seized large areas of Iraq and major cities.
In all but one case, the alleged executions took place while the soldiers were fleeing Isis as it advanced, HRW said.
The extremist group violently ascribes to Sunni Islam, whereas the majority of Iraqi soldiers are Shias, as is the central government.
Isis has shot and beheaded unknown numbers of captured soldiers, police officers, Shia religious minorities and members of militias who tried to resist them.
Joe Stork, the deputy Middle East director of HRQ said gunning down prisoners is an “outrageous violation of international law”.
He added: “While the world rightly denounces the atrocious acts of ISIS, it should not turn a blind eye to sectarian killing sprees by government and pro-government forces.”
The group called on an international commission to investigate the alleged executions and human rights breaches by all sides in the continuing Iraq crisis.
Iraq’s ethnic and religious groups are being pulled apart by the sectarian conflict, where Sunni Isis is pitted against the Shia Iraqi government and Kurds have taken control of an area in the north of the country.
Kurdish forces have continued to grow in strength, seizing two oilfields in northern Iraq and taking over operations from a state-run oil company on Friday.
Kurdish politicians have further weakened Prime Minister Nouri al-Maliki's government by suspending their participation after they were accused of harbouring extremists.
Hoshiyar Zebari, Iraq’s Foreign Minister, told Reuters the country is divided into three states, “Kurdish, a black state [Isis] and Baghdad".
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