Evidence indicates that militants from the Islamic State (IS) jihadist group executed more than 500 captives in Iraq earlier this year, Human Rights Watch said on Wednesday.
Around 1,700 soldiers surrendered to IS in June after its fighters seized second city Mosul and swept south towards Baghdad.
IS subsequently released photographs of dozens of men in civilian clothes apparently being executed by firing squad in desert areas, and said it had killed hundreds in total.
"Information from a survivor and analysis of videos and satellite imagery has confirmed the existence of three more mass execution sites, bringing the total to five, and the number of dead to between 560 and 770 men, all or most of them apparently captured Iraqi army soldiers," HRW said.
Signup to our newsletter and follow us on Facebook and Twitter!
"Another piece of this gruesome puzzle has come into place, with many more executions now confirmed," said Peter Bouckaert, emergencies director at the rights watchdog.
"The barbarity of the Islamic State violates the law and grossly offends the conscience."
The fate of the soldiers has caused widespread anger among their relatives, who stormed the parliament building in Baghdad on Tuesday, demanding to know their fate and for certain officers to be held accountable.
The hundreds of demonstrators assaulted some MPs as well parliamentary staff, smashed chairs in the cafeteria and started a sit-in the building's main chamber, an official said.
But the situation was later defused and the demonstrators departed, and parliament speaker Salim al-Juburi met with their representatives at his home, the official said.
Parliament held a lengthy session on the missing soldiers Wednesday, during which the acting defence minister and senior officers were questioned, with relatives of the soldiers in attendance.