Human Rights Watch said on Thursday that fighters inspired by Al-Qaeda had executed Iraqi soldiers en masse this month in Tikrit.
In mid-June, militants from the Islamic State of Iraq and the Levant (ISIS) posted photos online of what they claimed to be the bodies of dozens of captured security forces members they had executed.
"Analysis of photographs and satellite imagery strongly indicates that the Islamic State of Iraq and Syria conducted mass executions in Tikrit after seizing control of the city on June 11, 2014," Human Rights Watch said in a statement, using a different translation for the name of the same group.
It suggested the death toll was between 160 and 190 men in at least two locations between June 11 and June 14. ISIS had claimed to have killed 1,700 Shiite soldiers in Tikrit, once a bastion of late dictator Saddam Hussein.
HRW acknowledged that the number of victims may well be much higher than the bodies it found, as it underscored the challenge accessing the area.
"The photos and satellite images from Tikrit provide strong evidence of a horrible war crime that needs further investigation," said HRW emergencies director Peter Bouckaert.
The rights group located two of the trenches filled with bodies by cross-checking against ground features and landmarks in the photographs released by ISIL.
It also checked the information against satellite imagery from 2013 and photographs from Tikrit taken earlier that had been made publicly available.
Two trenches were at the same location, just steps from what was once Saddam's Water Palace. A third trench could not be located.
"During an armed conflict, the murder of anyone not taking an active part in hostilities, including members of armed forces who have laid down their arms and those in detention, is a war crime," HRW stressed.
"Murder, when systematic or widespread and committed as part of a deliberate policy of an organized group, can be a crime against humanity."
The rights group has previously documented other serious crimes by ISIS.
"ISIS is committing mass murder, and advertising it as well," Bouckaert said. "They and other abusive forces should know that the eyes of Iraqis and the world are watching."
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