Amnesty International says Iraq's Shiite militias have abducted and killed "scores" of Sunni civilians in retaliation for the onslaught of the Sunni militant Islamic State group.

The rights watchdog says these attacks are supported by Iraq's government, and that the Shiites are not prosecuted for their crimes.

Amnesty's senior adviser Donatella Rovera is accusing the Shiite-led government in Baghdad of "sanctioning war crimes and fueling a dangerous cycle of sectarian violence that is tearing the country apart."

Amnesty says the fate of many of the Sunni abductees remains unknown and that some captives have been killed even after their families paid ransoms of $80,000 and more.

The accusations, based on interviews with families and survivors, came in a new Amnesty report released Tuesday, entitled "Absolute Impunity: Militia Rule in Iraq."