Militants from the Islamic State in Iraq and Syria boasted on Twitter that they had executed 1,700 Iraqi government soldiers, posting gruesome photos to support their claim.

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.

If the claim is true, it would be the worst mass atrocity in either Syria or Iraq in recent years, surpassing even the chemical weapons attacks in the Syrian suburbs of Damascus last year, which killed 1,400 people and were attributed to the Syrian government.

The latest attack, if proved, would also raise the specter of the war in Iraq turning genocidal, particularly because the insurgents boasted that their victims were all Shiites. There were also fears that it could usher in a series of reprisal killings of Shiites and Sunnis, like those seen in the Iraq war in 2005-7.

The office of the Shiites’ supreme spiritual leader, Grand Ayatollah Ali al-Sistani, Saturday night issued what amounted to a revision of the ayatollah’s call to arms on Friday, apparently out of concern that it was misinterpreted by many as a call for sectarian warfare.

The statement, billed as “clarifying the position on taking up arms,” implored Iraqis, “especially those living in mixed areas, to exert the highest level of self-restraint during this tumultuous period.”

The claim of the mass execution appeared on a Twitter feed previously used for ISIS announcements, so whether or not the executions were genuine, the organization certainly intended to boast of them.

“We’re trying to verify the pics, and I am not convinced they are authentic,” said Erin Evers, the Human Rights Watch researcher in Iraq. “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.”

News was slow to circulate in Iraq, however, since the government last week blocked social network sites, including YouTube, Twitter and Facebook.

An Iraqi military intelligence official confirmed that the military was aware of the reported executions in Salahuddin Province, which includes the key city of Tikrit, but he did not know how many there were. He spoke on the condition of anonymity in line with his agency’s rules.

Col. Suhail al-Samaraie, head of the Awakening Council in Samarra, a pro-government Sunni grouping, also confirmed that officials in Salahuddin were aware of large-scale executions having taken place last week, but he did not know how many. “They are targeting anyone working with the government side, anyplace, anywhere,” he said. He said the insurgents were targeting anyone with a government affiliation, whether Sunni or Shiite.

One of those executed by the insurgents was a police colonel named Ibrahim al-Jabouri, a Sunni official in charge of the criminal investigation division in Tikrit, according to Mr. Samaraie.

A local journalist familiar with the Iraqi military in Salahuddin Province said the Fourth Iraqi Army Division had collapsed as the insurgents advanced last week, and 4,000 soldiers were believed to have been captured. Local reports said many of the victims were Sunnis as well as Shiites, he added.

A New York Times employee in Tikrit said by telephone that residents spoke of seeing hundreds of prisoners captured when they tried to flee Camp Speicher, a former American military base and airfield on the edge of Tikrit that was turned into an Iraqi training center. Those who were Sunnis were given civilian clothes and sent home; the Shiites were taken to the grounds of Saddam Hussein’s old palace in Tikrit, where they were said to be executed, their bodies dumped in the Tigris River, which runs by the palace compound.

The ISIS photographs appeared to have been taken at that location. However, the Times employee said he had not spoken to any witnesses who claimed to have seen the executions or bodies.

The still photographs uploaded on the ISIS Twitter feed were bloody and gruesome, showing the insurgents, many wearing black masks, lining up at the edges of what looked like hastily dug mass graves and apparently firing their weapons into groups of young men who were bound and packed closely together in large groups.

The photographs showed at least five massacre sites, with the victims lying in shallow mass graves with their hands tied behind their backs. The number of victims that could be seen in any of the pictures numbered between 20 and 60 in each of the sites, although it was not clear whether the photographs showed the entire graves. Some appeared to be long ditches.

The photographs showed the executioners flying the ISIS black flag, with captions such as “the filthy Shiites are killed in the hundreds,” “The liquidation of the Shiites who ran away from their military bases,” and “This is the destiny of Maliki’s Shiites,” referring to Prime Minister Nuri Kamal al-Maliki.

Many of the captions were viciously mocking toward the purported victims. In one photograph, showing 25 young men walking toward an apparent execution site, where armed, masked men awaited, the caption read, “Look at them walking to death on their own feet.”

And another showed a couple hundred prisoners, all of whom had been made to stand, bent over from the waist with their hands clasped behind their backs, as armed men guarded them. All were in civilian clothes, and the caption claimed they had jettisoned their uniforms. “They were lions in uniform, and now they are just ostriches,” it read.

Other photographs showed prisoners, mostly young men, stuffed in large numbers in dump trucks and pickup trucks. They appeared extremely frightened.

A senior Iraqi government official, speaking on the condition of anonymity because he was not authorized to make press statements, said news of the executions was slow to circulate because Twitter had been blocked. “I don’t doubt they are real, but 1,700 is a big number,” he said. “We are trying to control the reaction. They are trying to bring back the 2005 to 2006 days.” Sunni and Shiite militias engaged in a wave of tit-for-tat killings of civilians during that period, killing tens of thousands.

•Excerpted from New York Times. Photo shows a group of captured Iraqi soldiers being marched to their execution point by ISIS insurgents.

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