The images show gunman pointing weapons at men in civilian clothes, some with their hands over their heads, others lying down in a ditch. Some of the photos were captioned, "apostates heading to their hole of doom."
The New York Times reported Sunday that ISIS claimed on Twitter that its members had executed 1,700 Iraqi government soldiers.
If the claims are true, the mass execution would be the largest single atrocity since the U.S.-led invasion of Iraq in 2003, according to BBC News analyst Jim Muir.
Though the photos have yet to be independently verified, Iraqi military spokesman Lt Gen Qassim al-Moussawi said Sunday that he had heard of such an atrocity taking place.
The photos followed the State Department's announcement Sunday that it had relocated additional security personnel to the U.S. Embassy in Baghdad, while temporarily evacuating some staff members to more stable consulates or to neighboring Jordan.
Overall, a substantial majority of the U.S. Embassy presence in Iraq will remain in place and the Embassy will be fully equipped to carry out its national security mission.
With about 5,000 personnel, the U.S. Embassy in Baghdad is America's largest diplomatic post.