A rights group has said Iraqi security forces and Shia militia members have been involved in executing at least 255 prisoners in six cities and villages.
The New York-based Human Rights Watch (HRW) said on Saturday that Sunni prisoners were killed as Shia fighters and Iraqi soldiers fled advancing Islamic State rebels.
At least eight of those who were killed were boys under age 18, the group says.
According to HRW, five massacres of prisoners were documented between June 9 and 21 in Mosul, Tal Afar in northern Nineveh province, in Baquba and Jumarkhe in Diyala province, and in Rawa in Anbar province.
|Map: The Islamic State's (formerly ISIL) path through Iraq
The group's report cited evidence from witnesses, accounts of the attacks from anonymous police sources and interviews with family members of some of the victims.
"The mass extrajudicial killings may be evidence of war crimes or crimes against humanity," HRW said in its report, adding that it appeared the executions were to avenge "atrocities" by the Sunni Islamic State group.
"Gunning down prisoners is an outrageous violation of international law," said Joe Stork, the deputy Middle East director at HRW.
"While the world rightly denounces the atrocious acts of IS, it should not turn a blind eye to sectarian killing sprees by government and pro-government forces."
In most cases the prisoners were shot dead. In two cases, grenades were thrown inside prison cells of the victims, and in another case dozens of prisoners were set on fire.
The HRW quoted a Reuters news agency report that in a sixth attack, on June 23 in central Babil province, police executed 69 prisoners in their cells in the city of Hilla before transferring their bodies to Baghdad later that day.
Fighting in Haditha
In Saturday's other developments, Iraqi forces said they beat back an assault on Haditha in Anbar, strategic for its large nearby dam.
The attack on Haditha, located northwest of Baghdad on the road linking Sunni rebel-held western areas and the provincial capital Ramadi, located 115km west of Baghdad, began with mortar fire, police said.
Rebels travelling in vehicles, including some captured from security forces, then attacked from two sides but were kept from entering the town in fighting that left 13 fighters and four police officers dead, officers and a doctor said.
Previous attacks on Haditha were of a smaller scale and the capture of the dam by the Sunni rebels would raise the prospect of it being used to cut water or flood areas downstream, as happened earlier this year elsewhere in Anbar.
Iraqi officials were airlifting more volunteers to Ramadi to assist government forces, reports said.
About 2,500 volunteers arrived on Friday and were to be joined by another 1,500 on Saturday, General Rasheed Flayeh, commander of operations in Anbar, said.
The men were being ferried out to Ramadi from Baghdad by helicopter, Flayeh said.
The majority of volunteers were Shias who answered a call from the country's top Shia religious leader, Ayatollah Ali al-Sistani, to defend Iraq from the Islamic State group's advance.
Shia towns overrun
In Diyala, meanwhile, security forces and civilian volunteers on Saturday launched a push to retake rebel-held areas north of Muqdadiyah, a town on a main road to the provincial capital Baquba, a police captain said.
But in a setback for Iraqi government forces, rebels overran the Shia-majority towns of Al-Tawakul and Al-Zarkush in the province, displacing local residents, witnesses said.
In Jalawla, another Diyala town, Kurdish Peshmerga fighters began a major operation to expel rebels from areas they hold, a senior Kurdish officer said.
Major-General Hussein Mansur said Kurdish forces were using tanks and artillery in the battle, and had succeeded in retaking territory from the rebels.
Against this backdrop, Nickolay Mladenov, the UN envoy to Iraq, warned Iraqi politicians that "failure to move forward on electing a new speaker, a new president and a new government risks plunging the country into chaos".
"It will only serve the interests of those who seek to divide the people of Iraq and destroy their chances for peace and prosperity," he said on Saturday.