Iraqi extremists killed at least 160 captives in a major execution in the city of Tikrit earlier this month.
Number of dead bodies, though, could be much higher since a full investigation could not be done due to difficulty in locating bodies, said Human Rights Watch group.
Peter Bouckaert, emergencies director at the US based rights group said in a statement that the photos on the militant website and satellite images provide a clear evidence of gruesome violence which requires complete investigation.
According to The Independent, Powerful Shia cleric Moqtadaal-Sadr pledged to destroy the ground under Sunni militants, which can inflict further unrest in the region.
Sadr's statement reportedly came out as security forces tried to repel violence in adversely affected towns.