The Human Rights Watch released a statement on Tuesday confirming the locations of three mass grave sites where Iraqi troops were executed after the Islamic State of Iraq and the Levant (ISIS) overran north Bhagdad's military base in June.

The recently discovered mass graves are located near the Camp Speicher air base in Tikrit. New evidence also confirms that the death toll has tripled from the previously approximated total.

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With the new Human Rights Watch evidence, the total number of dead rose from 560 to 770 Iraqis, most of whom were believed to be captured soldiers during the Tikrit siege in June 11.

The black-cladded ISIS that has been sweeping Iraq and Syria had reportedly buried between 160 to 190 Iraqi soldiers at the two previously identified execution sites.

"Another piece of this gruesome puzzle has come into place, with many more executions now confirmed," said Peter Bouckaert, emergencies director of Human Rights Watch. He added that the barbarism the Islamic State grossly represents "violates the laws and offends the conscience."

After running the recently released Sotloff beheading footage, including satellite imagery and eyewitness evidence, the Human Rights Watch identified two more locations, and pegged the death toll at 285 to 440 men.

The location of the third execution site has been approximated based on the accounts provided by the only known survivor of the massacre, Ali Hussein Kadhim.

Kadhim, 23, was captured on June 12 along with thousands of other Iraqi soldiers. In a gripping interview released by CNN, Kadhim described the harrowing details of his survival.

Human Rights Watch reported that Kadhim and thousand other Iraqi soldiers were detained in shipping containers and were later shot after marching along the banks of the Tirgris river. Kadhim pretended to be dead among a pile of slain soldiers and escaped after the mass execution.

There were at least 1,700 Shiite soldiers killed between June 12 to 13, reported the New York Times. The largest of the executions, according to the Human Rights Watch, was at Tikrit's presidential palace where about 200 to 400 bodies were killed. The death toll is expected to rise as the Watch continues to gather more evidence.

TagsISIS, mass execution, Human Rights Watch, mass graves