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."