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Associated Press
If I could apologize to each American, one by one, I would, because Peter died in Syria, while he was helping the Syrian people. Those who killed him claimed to have done it in the name of Islam. I am a Muslim and am from Syria. ... (His killers) are not Muslims. —Burhan Agha, friend

BEIRUT — The Islamic State group released a video Sunday of a masked, black-clad militant standing over a severed head it claimed is American aid worker Peter Kassig, a former Army Ranger, as well as the mass beheadings of a dozen Syrian soldiers.

The White House confirmed the authenticity of the video. In a statement issued as he flew back to Washington after a trip to the Asia Pacific region, President Barack Obama said the group "revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction."

With Kassig's death, the Islamic State group has killed five Westerners it was holding. Britons David Haines, a former Air Force engineer, and Alan Henning, a taxi driver from northwest England, were beheaded, as was U.S. reporter James Foley and American-Israeli journalist Steven Sotloff.

Unlike previous videos of slain Western hostages, the footage released Sunday did not show the purported decapitation of the American or the moments leading up to his death.

"This is Peter Edward Kassig, a U.S. citizen ... who fought against the Muslims in Iraq," said the militant, who spoke with a British accent distorted in the video, appeared on websites used in the past by the Islamic State group, which now controls a third of Syria and Iraq.

After his capture in eastern Syria on Oct. 1, 2013, while delivering relief supplies for the aid group he founded, Kassig had converted to Islam and took the name Abdul-Rahman Kassig.

Obama said Kassig "was a humanitarian who worked to save the lives of Syrians injured and dispossessed" by war.

The president said the 26-year-old Indianapolis man "was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity."

Obama offered prayers and condolences to Kassig's family. "We cannot begin to imagine their anguish at this painful time," he said.

Kassig's family said in a statement that it preferred "our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause."

The footage released Sunday identifies the militants' location as Dabiq, a town in northern Syria that the Islamic State group uses as the title of its English-language propaganda magazine and where they believe an apocalyptic battle between Muslims and their enemies will occur.

The high-definition video also showed the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits. The black-clad militant warns that U.S. soldiers will meet a similar fate.

"We say to you, Obama: ... You claim to have withdrawn from Iraq four years ago," the militant said. "Here you are: You have not withdrawn. Rather, you hid some of your forces behind your proxies."A U.S.-led coalition is targeting the Islamic State group in airstrikes, supporting Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.

Previous videos featured a militant with a British accent that the FBI says it has identified, though it hasn't named him publicly.

Kassig served in the U.S. Army's 75th Ranger Regiment, a special operations unit, and deployed to Iraq in 2007. After being medically discharged, he returned to the Middle East in 2012 and formed an aid organization, Special Emergency Response and Assistance, to aid Syrian refugees. A certified EMT, Kassig had provided care to wounded Syrians before he was captured in eastern Syria.

Burhan Agha, a Syrian who worked with Kassig in Lebanon, and who moved to Switzerland seeking asylum, wept when recounting his friend.

"If I could apologize to each American, one by one, I would, because Peter died in Syria, while he was helping the Syrian people," Agha told the AP by telephone. "Those who killed him claimed to have done it in the name of Islam. I am a Muslim and am from Syria. ... (His killers) are not Muslims."

British Prime Minister David Cameron said he was "horrified by the cold-blooded murder," saying that the Islamic State group had "again shown their depravity."

In previous videos showing the beheadings of the two American journalists and two British aid workers, the hostages were shown kneeling in orange jumpsuits as they were forced to make speeches before their killer lifted a knife to their throats.

The latest video did not show the person identified as Kassig being beheaded. And unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else. It also had lingering close-ups on some militants' exposed faces, a few or whom appeared to be foreigners.

The Islamic State group still holds other captives, including the British photojournalist John Cantlie, who has appeared in several videos delivering statements for the group, likely under duress, and a 26-year-old American woman captured last year in Syria while working for aid groups. U.S. officials have asked that the woman not be identified out of fears for her safety.

The video appeared to be part of continuous efforts to strike at the U.S. as airstrikes continue against the group. The airstrikes began in August in Iraq and spread to Syria the following the month.

They appear to have at least halted the sweeping advance of the militants, who have seized nearly one-third of Syria and Iraq, where they impose their violent interpretation of Sharia law.

The strikes have also led to a grueling stalemate in the Syrian border town of Kobani, where advancing militants caused the flight of around 200,000 Syrian Kurds.

The video came two days after a recording by the group's leader, Abu Bakr al-Baghdadi, was uploaded to the internet. The militant leader also warned that the U.S.-led coalition's campaign had failed and it would eventually have to send ground troops into battle in Thursday's recording.

The group has declared a self-styled Islamic caliphate in areas under its control, which it governs according to its violent interpretation of Shariah law, including massacring rebellious tribes and selling women and children of religious minorities into slavery. The group's militants have also beheaded and shot dead hundreds of captives, mostly Syrian and Iraqi soldiers, including the some half-dozen who appeared on the video released Sunday.

Associated Press writers Julie Pace in Brisbane, Australia, David Aguilar in Detroit, Jon Gambrell in Cairo, Vivian Salama in Baghdad and Josh Lederman aboard the Air-Force One contributed to this report.