An international rights group says Shiite militias allied with Iraqi security forces have escalated a campaign of abuse against Sunni residents in recent months.

New York-based Human Rights Watch said in a statement issued Sunday that the militiamen, who are part of the fight against the Islamic State group, have begun driving Sunni families from their homes, kidnapping or summarily executing them in some cases.

The report said the abuses are taking place mainly in areas that were seized from the Islamic State group — which holds about a third of Iraq and Syria.

HRW says the militias — mainly volunteers who answered the call-to-arms from Shiite clerics — are growing more brutal, stoked by a desire for revenge against the Sunni extremists who have frequently butchered and attacked Shiites.