Iraqi security forces and government-affiliated militias appear to have executed at least 255 prisoners since 9 June, a human rights group says.
The killings appeared to be retaliation for attacks by the jihadist Islamic State in Iraq and the Levant (Isis), Human Rights Watch said in a statement.
The prisoners were all Sunni Muslims, while the majority of security forces and militia were Shia, they added.
Meanwhile, Iraqi Kurds have reportedly taken over two oilfields in the north.
Most of the executions took place as Iraqi forces fled advancing Isis fighters, HRW said in a statement.
The killings took place in six Iraqi villages: Mosul, Tal Afar, Baquba, Jumarkhe, Rawa and Hilla, HRW reported.
"The mass extrajudicial killings may be evidence of war crimes or crimes against humanity, and appear to be revenge killings for atrocities by Isis," the statement said.
Last month, Isis insurgents seized huge swathes of north-western Iraq. The group has gained a reputation for brutal rule in the areas that it controls.
Joe Stork, HRW's deputy Middle East director, said: "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 HRW statement added that the executions, which it documented based primarily on interviews with eyewitnesses and officials, "may be evidence of war crimes or crimes against humanity".
The current conflict in Iraq has acquired strong sectarian overtones. The Shia-led government is struggling against predominantly Sunni Isis fighters, and other Sunni rebel groups.
Meanwhile, reports say the Iraqi Kurds have taken over the northern Bai Hassan and Kirkuk oilfields, amid a growing dispute with the government in Baghdad.
The Iraqi oil ministry has condemned the seizure, saying it expects Kurdish fighters to "support security forces in confronting terrorist groups rather than using the conditions to raid and occupy oil fields".
THREE WAY DISPUTE
Kurdish MPs have also withdrawn from Iraq's central government, after Iraqi Prime Minister Nouri Maliki accused the Kurds of harbouring extremists.
Iraqi Foreign Minister Hoshiyar Zebari, who is himself a Kurdish politician, told Reuters news agency on Friday that the country risked division if an inclusive government was not formed soon.
"The country is now divided literally into three states - Kurdish, a black state [Isis] and Baghdad," he said.
Kurdish forces moved into areas of north-western Iraq last month as the Iraqi army retreated from the Isis advance.
The Kurds have since declared plans to hold a referendum on independence in the areas seized, escalating tensions with Iraq's central authorities.
It is a three way dispute between the Kurds, the Sunnis and the Shia that is getting deeper and could break Iraq apart, the BBC's Mark Doyle in Baghdad reports.
More than a million people have fled their homes as a result of the fighting in recent months.