Shiite paramilitary forces have been repeatedly accused of abuses including summary executions, kidnappings and destruction of property in the course of the war against the Islamic State jihadist group.
Following January 11 bombings claimed by IS in Muqdadiyah, a town northeast of Baghdad, militiamen attacked "Sunnis as well as their homes and mosques, killing at least a dozen people and perhaps many more," HRW said, citing local residents.
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"Again civilians are paying the price for Iraq’s failure to rein in the out-of-control militias," Joe Stork, HRW's deputy Middle East director, said in a statement.
Killing civilians, looting and unjustified destruction of property during an armed conflict "are serious violations of international humanitarian law" and "may amount to war crimes," the rights group said.
Iraq turned to Shiite militia forces in 2014 to help counter an IS onslaught that overran large areas north and west of Baghdad, and they played a key role in halting the jihadist advance and later pushing them back.
But they have also carried out repeated abuses during the conflict that ultimately feed mistrust of the government and are harmful to Baghdad's efforts to reassert and maintain control in recaptured areas.