Human Rights Watch has said Sunni insurgents had executed between 160 and 190 Iraqi soldiers en masse this month in Tikrit.
In mid-June, insurgents 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 11 June, 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 11 and 14 and June.
ISIS, also known as ISIL, had claimed to have killed 1,700 Shia 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 ISIS.
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 organised 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," Mr Bouckaert said.
"They and other abusive forces should know that the eyes of Iraqis and the world are watching."
Meanwhile, US intelligence about the Islamist insurgent offensive in Iraq is improving but it could take weeks to complete a detailed picture of the threat, US officials have said.
They also said that any possible US air attacks do not appear imminent.
Last week's announcement that up to 300 US military advisers were being sent to Baghdad and the earlier movement of an aircraft carrier, a cruiser and a destroyer into the Gulf prompted speculation of impending military action against ISIS.
The US intelligence picture is being filled in with information from flights by about 30 to 35 manned surveillance planes and drones flying over the country daily.
US officials said this would be further boosted by the opening on Wednesday of a joint Iraq-US operations centre in Baghdad staffed by about 90 military personnel.
It will take time, the officials said, to build a detailed picture of ISIS's deployments, intentions and weapons stockpile, which has grown considerably since its black-clad forces overran Iraqi government arsenals in the last few weeks.
Another US official, who declined to be identified, said the patchy nature of current intelligence on ISIS's activities would not necessarily rule out early limited US air strikes should specific targets be identified.
US President Barack Obama has been reluctant to engage in the sectarian conflict.
The president has said the emphasis at this stage is on pressuring the Shia-dominated leadership in Baghdad to build an inclusive government that brings in Sunni and Kurdish factions to create a united front against ISIS's Sunni fighters.