IRBIL, Iraq -- The United States announced Sunday it was removing an undisclosed number of workers from the U.S. Embassy in Baghdad in a tacit acknowledgment the situation in the Iraqi capital had become unpredictable and violence seemed likely.

The announcement came as the Islamic State of Iraq and Syria claimed to have executed 1,700 Iraqi soldiers, all of them Shiite Muslims who'd been captured last week.

In a statement, the U.S. State Department did not reveal how many of its workers were being moved out of the embassy compound, saying that "overall, a substantial majority of the U.S. Embassy presence in Iraq will remain in place." But even if a majority remains at the compound, the evacuation could mean the departure of hundreds of workers. The Baghdad embassy is the United States' largest, with 5,300 U.S. government employees.

A separate announcement from the Defense Department said the evacuation of embassy personnel was "being facilitated aboard commercial, charter and State Department aircraft as appropriate." The statement said U.S. military "airlift assets" were "at the ready," but suggested the State Department had not requested military assistance in the evacuation.

Pentagon spokesman Navy Rear Adm. John Kirby said the U.S. military also sent a "small number" of Defense Department personnel to assist "State Department security assets in Baghdad to help ensure the safety of our facilities."

The decision to evacuate Americans from the huge embassy compound came as the likelihood of sectarian and ethnic violence in Baghdad grew. Fighters allied with the Sunni Muslim ISIS vowed to press their lightning advance across northern and central Iraq into the capital.

Earlier Sunday, ISIS posted photos on the Internet that it said depicted the execution of hundreds of members of the Iraqi security forces taken prisoner when ISIS overran the Iraqi city of Tikrit last week.

A statement accompanying the photos claimed the men were all Shiite Muslims -- an assertion sure to inflame Iraq's emotion-filled sectarian divisions that in recent days have inspired thousands of Shiite men to flock to Baghdad to volunteer to help the army counter ISIS's push into the capital and nearby cities.

ISIS, which now controls much of Anbar province in the west and Nineveh province in the north and has pushed into Salahuddin and Diyala provinces just north of Baghdad, considers Shiite Muslims to be heretics subject to death. The feud between Sunnis and Shiites dates to A.D. 680, when a Sunni army beheaded one of the most revered figures in Shiite history at the Battle of Karbala. Recent ISIS statements have urged its followers to march to Karbala.

In its statement accompanying the photos, ISIS said it had executed 1,700 Shiite prisoners in Tikrit, a claim that could not be verified. ISIS also said it had released 2,500 Sunni prisoners after receiving promises the men would repent and no longer fight for the government.

An Iraqi military spokesman told the Associated Press the pictures appeared to have been taken after a former U.S. military facility in Tikrit fell to ISIS forces. The spokesman, Lt. Gen. Qassim Moussawi, did not comment, however, on whether 1,700 had been confirmed killed, as the group claims.

The photos showed dozens of young men bound in the back of trucks, then being led into open fields and executed by masked men firing assault rifles. Most of the men appeared to be wearing civilian clothes.

The caption on one photograph, which showed a dozen young men with their heads bowed and hands bound behind their backs while being menaced by masked gunmen, said the Iraqi Army's "lions had been turned into ostriches." Other captions described similar scenes as "the apostates being led to their doom."

The decision to spare Sunni prisoners -- if true -- undoubtedly stems from a desire by ISIS to maintain good relations with the large number of Sunni tribes in central and western Iraq that have joined its offensive, fueled by rising anger at Shiite Prime Minister Nouri al Malaki, whose been accused of sectarian discrimination and brutality.

In addition to ISIS, a mix of tribal fighters and supporters of Saddam Hussein, the Iraqi strongman who was deposed by the Americans in 2003 and executed by the Shiite-led government that came to power afterward, have joined the fight, angered at what they say is sectarian discrimination and brutality by Shiite Prime Minister Nouri al Malaki's government.

Highlighting the sectarian nature of the conflict, which has threatened the future of the modern Iraqi state, was Malaki's decision -- backed by Shiite religious figures -- to call upon Shiite militias that mostly had been disbanded after the sectarian civil war that followed the American-led invasion. It has also drawn the attention of Shiite-ruled Iran, which appears willing to send aid and expertise, and perhaps deploy ground troops to help protect the regime.

The Obama administration is considering aid to the Iraqis, including possible airstrikes. But President Barack Obama conditioned any assistance on Maliki and other politicians putting aside their differences.