Iraqi officials on Sunday confirmed the authenticity of photos released by the Islamic State in Iraq and Syria, or ISIS -- the ultra-conservative Islamic militant group that has swept through much of western Iraq since last week -- which showed the massacre of nearly 1,700 Shiite soldiers.

Lt. Gen. Qassim al-Moussawi, Iraq’s chief military spokesperson, told Associated Press, on Sunday that the military examined the photos released on the group’s website, and confirmed that nearly 170 soldiers were shot to death after they were captured by the Sunni Islamic group in Tikrit. The captions on the photos reportedly said “hundreds have been liquidated," but the U.S. did not confirm the validity of the photos.

Several hundred volunteers have joined hands with Iraq's military to beat back ISIS, which has advanced rapidly by taking control of several cities in a short span of a few days, and has now set its sight on the capital, Baghdad.

"Baghdad International Airport has been struck by mortar rounds and rockets, and the Mosul International Airport has been the target of militant assault," a travel warning released Sunday by the State Department said, according to the AP.

Meanwhile the U.S., which decided to move its staff out of its embassy in Baghdad, began efforts to increase security to protect Americans in the city.

“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," Jen Psaki, State Department spokesperson, said Sunday, according to the AP.