As the U.S. expands its war on the Islamic State (ISIS), rising Shi'a militias within Iraq—many of them armed and backed by the U.S.-allied Iraqi government—are "taking advantage" of a climate of violence and impunity to abduct, execute, and disappear scores of Sunni civilian men, according to a report released Tuesday by Amnesty International.

"By granting its blessing to militias who routinely commit such abhorrent abuses, the Iraqi government is sanctioning war crimes and fueling a dangerous cycle of sectarian violence that is tearing the country apart," said Donatella Rovera, Amnesty International’s Senior Crisis Response Adviser.

Entitled Absolute Impunity: Militia Rule in Iraq, the investigation is based on information obtained through six weeks of field research in central and northern Iraq, including meetings with victims, witnesses, doctors, NGOs, and lawyers.

Increasingly powerful Shi’a militias in Baghdad, Samarra and Kirkuk are abducting Sunni civilian men from homes, jobs, and checkpoints and then killing and disappearing them, the report finds. Researchers uncovered evidence of summary executions, including reports from Health Ministry workers that they are receiving "scores" of dead bodies found handcuffed and shot in the back of the skull. Numerous other victims "are still missing, their fate and whereabouts unknown, weeks and months after they were abducted," the report states.

The relative of one victim from Kirkuk told Amnesty, "I have lost one son and don’t want to lose any more. Nothing can bring him back and I can’t put my other children at risk. Who knows who will be next?"

In some cases, families and loved ones paid ransoms, but the civilian was killed anyway. Salem, a 43-year-old business man, was one such victim. He was abducted from his factory in al-Taji, north of Baghdad, during the afternoon. His family paid a ransom of $60,000, but to no avail. Two weeks later, he turned up at a morgue, his head smashed and his hands cuffed behind his back.

The mother of another victim told Amnesty investigators, "I begged friends and acquaintances to lend me the ransom money to save my son, but after I paid they killed him and now I have no way to pay back the money I borrowed, as my son was the only one working in the family."

Some of these killings are in revenge for ISIS attacks on Shi'a civilians, while in other cases, the aim is to extort money, according to the report.

Many of these militias collaborate directly with Iraqi military forces, and some boast tens of thousands of fighters—functioning as small armies yet not subject to regulation or oversight. Included among them is the Mahdi Army, founded by cleric Moqtada al-Sadr after the 2003 U.S. occupation of Iraq.

The report charges that the Iraqi government that many of these militias are allied with is itself guilty of abuses. While the investigators focus on Shi'a militias, they also note evidence of the torture, mistreatment, and unlawful killings of people detained by Iraqi government forces.