Wielding the threat of sectarian slaughter, Sunni Islamist militants claimed on Sunday that they had massacred hundreds of captive Shiite members of Iraq's security forces, posting grisly pictures of a mass execution in Tikrit as evidence and warning of more killing to come.
Even as anecdotal reports of extrajudicial killings around the country seemed to bear out the Islamic State in Iraq and Syria's intent to kill Shiites wherever it could, Iraqi officials and some human rights groups cautioned that the militants' claim to have killed 1,700 soldiers in Tikrit could not be immediately verified.
But with their claim, the Sunni militants were reveling in an atrocity that if confirmed would be the worst yet in the conflicts that roil the region, outstripping even the poison gas attack near Damascus last year.
In an atmosphere where there were already fears that the militants' sudden advance near the capital would prompt Shiite reprisal attacks against Sunni Arab civilians, the claims by ISIS were potentially explosive. And that is exactly the group's stated intent: to stoke a return to all-out sectarian warfare that would bolster its attempts to carve out a Sunni Islamist caliphate that crosses borders through the region.
The sectarian element of the killings, and reports late Sunday that the city of Tal Afar, west of Mosul, had also fallen, may put more pressure on the Obama administration to aid Iraq militarily. In fact, the militants seemed to be counting on it. A pronouncement on Sunday by the group's leader, Abu Bakr al-Baghdadi, had a clear message for the United States: "Soon we will face you, and we are waiting for this day."
The group's announcement was made in a series of gruesome photographs uploaded to an ISIS Twitter feed and on websites late on Saturday night. Some showed insurgents, many wearing black masks, lining up at the edges of what looked like shallow mass graves and apparently firing their weapons into young men who had their hands bound behind their backs and were packed closely together in large groups.
The photographs showed what appeared to be seven massacre sites, although several of them may have been different views of the same sites. In any one of the pictures, no more than about 60 victims could be seen and sometimes as few as 20 at each of the sites, although it was not clear if the photographs showed the entire graves.
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The militants' captions seemed tailor-made to ignite anger and fear among Shiites. "The filthy Shiites are killed in the hundreds," one read. "The liquidation of the Shiites who ran away from their military bases," read another, and, "This is the destiny of Maliki's Shiites," referring to Prime Minister Nuri Kamal al-Maliki.
Many of the captions mocked the victims. In one photograph, showing a group of 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."
So far, Iraq's majority Shiites were not rising to the bait. Grand Ayatollah Ali al-Sistani, their supreme religious leader, issued a statement on Saturday calling for all groups to "exert the highest possible level of self-restraint during this tumultuous period." And there was little immediate public reaction to the ISIS claims in Baghdad or other southern Iraqi cities.
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 and other social media sites had been blocked for days. "I don't doubt they are real, but 1,700 is a big number," he said. "We are trying to control the reaction."
Acutely aware of the potential for retaliatory violence, some government officials who had heard about the ISIS claims took pains to play them down, confirming only that some executions had taken place in Tikrit, but not on a large scale.
One Iraqi security official claimed that no more than 11 bodies of executed soldiers were recovered from the Tigris River downstream from the execution site, a group of six and a group of five, although he confirmed that 800 soldiers had been taken prisoner in the area. He also reported finding 17 bodies washed up against a dam near Samarra, another city the militants are fighting for. But he said, "There is no such superstitious number as 1,700 people executed."
An official statement posted on the Ministry of Defense's website denied the executions had taken place at all.
Still, other officials and human rights representatives, while cautioning that they could not confirm the full 1,700 number being claimed, said that ISIS had shown no compunctions against hunting Shiites. And they reiterated that such horrific claims would go to further the group's intent to sow chaos.
"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."
Col. Suhail al-Samaraie, head of the Awakening Council in Samarra, a pro-government Sunni grouping, confirmed that officials in Salahuddin Province were aware that large-scale executions had taken place, but did not know how many. "They are targeting anyone working with the government side, any place, anywhere," he said. He said the insurgents were targeting both Sunnis and Shiites, anyone with a government affiliation, but claiming for propaganda reasons that the victims were all Shiites.
A New York Times employee in Tikrit said local residents saw hundreds of Iraqi military personnel captured when they tried to flee Camp Speicher, a former American military base and airfield now used as an air force training center on the edge of Tikrit. It is still in government hands.
Most of those captured were air force cadets, the employee said. Those who were Sunnis were given civilian clothes and sent home; the Shiites were marched and trucked off to the grounds of Saddam Hussein's old palace in Tikrit, where they reportedly were executed. He added that the bodies had been dumped in the Tigris River, which runs by the palace compound.
The ISIS photographs appeared to have been taken at that location, the employee said. However, he said he had not spoken to any witnesses who claimed to have seen the executions or the victims' bodies.
Ryan C. Crocker, a former ambassador to Iraq and a critic of America's 2011 withdrawal from that nation after the two countries failed to sign a mutual security pact, said the atrocity claims, proven or not, made it more urgent than ever for Washington to become involved.
"What this administration has to do is get John Kerry on a plane right now, like we did when I was there, and sit down with Shia, Sunni and Kurdish leaders and help them get to a position of declared national unity. Iraqis have to stand together now," Mr. Crocker said. Regarding the massacres, he said, "Whatever it is, however many people, it's clearly an effort to ignite an Iraqi civil war."
Political analysts here mostly agreed about the militants' intent. "The problem now is that you are dealing with emotions and ISIS is trying to provoke the other side to take revenge," said Ameer Jabbar al-Sa'aedi, a Baghdad-based analyst. "There are extremists among the Shia, too, and if they respond, they could begin killing and not exclude anyone. It would be just like what happened in 2006."
Even though Ayatollah Sistani's statement over the weekend was intended to call for restraint on the part of Shiites, it came after his call just a day before for every Iraqi to take up arms to support the government.
That appeal was expected to greatly accelerate the formation of volunteer groups to supplement Shiite militias — nominally to fight alongside the Iraqi Army. But during the worst of the sectarian bloodletting in Iraq, from 2005 through 2007, some such Shiite groups were deeply involved in violence that was killing as many as 1,000 civilians each week.
One militia leader, Abu Bakr al-Zubaidi, from a group called Asaib Al Haq, a hard-line offshoot of the Mahdi Army militia, said he was not surprised to hear of the executions.
"ISIS regards Shia as their eternal enemy, and they will kill whoever falls in their hands who is Shia, whether they are soldiers, grocers or even singers," he said. "Our response to that is there will not be any living ISIS prisoner."