The Islamic insurgents that captured two major cities in northern Iraq last week have claimed on Twitter to have executed 1,700 Iraqi government soldiers, supporting their claim with graphic photos.

The images appear to show the militants from the Islamic State of Iraq and Syria loading the soldiers onto flatbed trucks before forcing them to lie face-down in a ditch with their arms tied behind their backs. The final images appear to show the lifeless bodies of the captives.

The New York Times reports that the claim of the mass execution was posted to a Twitter feed previously used for ISIS announcements, which proves intent, but that the authenticity of the photos is still in question:

The authenticity of the photographs and the insurgents' claim could not be verified, and Iraqi government officials initially cast doubt on whether such a mass execution took place. There were also no reports of large numbers of funerals in the Salahuddin Province area, where the executions were said to have been conducted.

Erin Evers, the Human Rights Watch researcher in Iraq, also expressed doubt:

"We're trying to verify the pics, and I am not convinced they are authentic. ... As far as ISIS claiming it has killed 1,700 people and publishing horrific photos to support that claim, it is unfortunately in keeping with their pattern of commission of atrocities, and obviously intended to further fuel sectarian war."

The Times reports that if the claim is true, it would surpass last year's chemical weapons attacks in Damascus, which killed 1,400, as the worst mass atrocity in either Syria or Iraq in recent years.

[image via AP]