Iraqi security forces have indulged in “mass extrajudicial killings” of at least 255 prisoners since June 9, Human Rights Watch, or HRW, charged Friday.

In a statement, the international human-rights organization said the executions took place when Iraqi forces were fleeing Islamic State of Iraq and Syria, or ISIS, militants. It added that while the “vast majority” of the security forces and militias were Shiites, the prisoners were all Sunnis.

Because the Shiite-led Iraqi government is pitched against a Sunni extremist group, the conflict in the country has acquired strong sectarian overtones. The fighting has divided the country into three factions -- the majority Shiites, the minority Sunnis and the Kurds, who have demanded a referendum on Kurdish independence.

The killings reportedly took place in six towns -- Baquba, Hilla, Jumarkhe, Mosul, Rawa and Tal Afar  -- located in the north of the country. HRW said it had documented five massacres of prisoners between June 9 and June 21 and that the Reuters news agency reported on another one happening June 23.

“In each attack, statements by witnesses, security forces and government officials indicate that Iraqi soldiers or police, pro-government Shia militias, or combinations of the three, extrajudicially executed the prisoners, in nearly all cases by shooting them. In one case the killers also set dozens of prisoners on fire, and in two cases they threw grenades into cells,” HRW said in its statement.  

The Iraqi government and security forces have been battling an increasingly belligerent ISIS in the north of the country since May. In June, ISIS successfully seized huge swathes of northwestern Iraq and declared it a caliphate.

Joe Stork, HRW’s deputy Middle East director, said the executions were an “outrageous violation of international law” and may be evidence of war crimes. “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,” Stork said.

Last month, the United Nations high commissioner for human rights, Navi Pillay, issued a statement saying she was “deeply disturbed” by reports of “summary executions, rape, reprisal killings and shelling of civilians” by ISIS militants. She added that the conduct of ISIS fighters would remain under “particular scrutiny.”

HRW demanded an investigation into “violations of the laws of war and international human-rights law by all sides” by an international commission of inquiry so that those responsible would be held accountable.