BAGHDAD — Wielding the threat of sectarian slaughter against Iraq’s government, Sunni Islamist militants marching toward the capital Sunday claimed they had killed hundreds of Shi’ite members of the Iraqi security forces, posting pictures of a mass execution in Tikrit as proof, and warning of more such sectarian attacks.
The militant group known as the Islamic State of Iraq and the Levant boasted on Twitter that its forces had executed 1,700 Iraqi government soldiers. The authenticity of the photographs and the insurgents’ claim could not be verified.
Lieutenant General Qassim al-Moussawi, Iraq’s chief military spokesman, said experts had confirmed that the photos were genuine and showed at least 170 soldiers being shot to death after their capture last week in Salahuddin province, which includes Tikrit, the Associated Press reported.
If the group’s claim about the number of men killed is true, it would be the worst mass atrocity in either Iraq or Syria in recent years, surpassing even the chemical weapons attacks in the suburbs of Damascus last year, which killed 1,400 people and were attributed to the Syrian government.
The attack could also raise the specter of the war in Iraq turning genocidal, particularly because the insurgents claimed that their victims were all Shi’ites.
Security at the US Embassy in Baghdad was strengthened Sunday and some staff members were sent elsewhere in Iraq and to neighboring Jordan. State Department spokeswoman Jen Psaki said much of the 5,500-member embassy staff will remain.
A series of bomb explosions in Baghdad killed at least 19 people and wounded more than 40 Sunday, police and hospital officials said.
While the capital was not in any immediate danger of falling to the militants, food prices have risen because of transportation disruptions and the city is under a 10 p.m. curfew. Traffic was thin earlier Sunday and there were few shoppers in commercial areas.
Defense Secretary Chuck Hagel ordered the aircraft carrier USS George H.W. Bush from the northern Arabian Sea to the Persian Gulf, as President Obama considered possible military options.
Hagel’s press secretary, Rear Admiral John Kirby, said the move will give Obama additional flexibility if military action were required to protect American citizens and interests in Iraq.
Accompanying the carrier was the guided-missile cruiser USS Philippine Sea and the guided-missile destroyer USS Truxtun.
The ships, which arrived in the gulf late Sunday, carry Tomahawk missiles that could reach Iraq. The Bush also has fighter jets that could easily reach Iraq.
In Washington, leading Republican lawmakers called on the Obama administration to take immediate action, in conjunction with allies in the Middle East and possibly even with Iran, to halt the surprisingly swift progress of extremist forces in Iraq.
Neither Representative Mike Rogers, a Michigan Republican and chairman of the House Intelligence Committee, nor Representative Michael T. McCaul, Republican of Texas and chairman of the House Homeland Security Committee, suggested that Obama deploy ground troops.
But Rogers and Senator Lindsey Graham, Republican of South Carolina, said that airstrikes might be necessary.
Speaking to volunteers south of Baghdad, Prime Minister Nouri al-Maliki vowed Sunday to regain territory captured last week by the militant group. ‘‘We will march and liberate every inch they defaced, from the country’s northernmost point to the southernmost point,’’ he said.
The office of the Shi'ites’ supreme spiritual leader, Grand Ayatollah Ali al-Husseini al-Sistani, Saturday night issued what amounted to a revision of the ayatollah’s call to arms 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.”
Erin Evers, the Human Rights Watch researcher in Iraq, said her group was trying to verify the authenticity of the execution photos.
“It is unfortunately in keeping with their pattern of commission of atrocities, and obviously intended to further fuel sectarian war,” she said, referring to the Islamic State of Iraq and the Levant, also known as the Islamic State of Iraq and Syria.
News of the killings was slow to circulate in Iraq, since the government last week blocked social network sites, including YouTube, Twitter, and Facebook.
Colonel Suhail al-Samaraie, head of the Awakening Council in Samarra, a progovernment Sunni grouping, said 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,” and not all the victims were Shi’ites, he said.
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, Samaraie said.
A local journalist 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.
A New York Times employee in Tikrit said residents spoke of seeing hundreds of prisoners captured when they tried to flee Camp Speicher, a former US 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 Shi'ites 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 Islamic State of Iraq and the Levant photographs appeared to have been taken at that location.
The still photographs uploaded on Twitter 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 Islamic State of Iraq and the Levant black flag, with captions such as “the filthy Shi'ites are killed in the hundreds,” “The liquidation of the Shi'ites who ran away from their military bases,” and “This is the destiny of Maliki’s Shi'ites,” referring to the prime minister.
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.”