A graphic video purportedly released by the militant Islamic State (IS) group shows that the Sunni extremist group has beheaded U.S. aid worker Peter Kassig, also known as Abdul-Rahman Kassig, and more than a dozen men described as Syrian soldiers.
The 16-minute recording shows a masked militant claiming to have beheaded Kassig and standing over a severed head.
The United States subsequently confirmed the death of Kassig, and President Barack Obama offered prayers and condolences to Kassig's parents and other relatives.
"Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," Obama said in a statement released aboard Air Force One as he flew back to the United States from an Asia tour.
The United States has conducted air strikes against IS targets and conducted aid drops to their victims.
The video also depicts the simultaneous beheadings of at least 15 men described as Syrian military personnel.
It shows masked militants marching the prisoners by a wooden box of long military knives, each taking one as they pass, before forcing their victims to kneel in a line and decapitating them. The militants warn that U.S. soldiers will meet a similar fate.
Kassig, a 26-year-old former Army Ranger who later joined humanitarian efforts and converted to Islam, was helping victims of the Syrian civil war when he was captured last year.
The video was posted on websites used by the IS in the past.
The National Security Council, which advises the U.S. president, said earlier that if the video was confirmed, "we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends."
'World Needs Him'
Kassig's family said that after leaving the army, Kassig became an aid volunteer. He was said by friends to have converted to Islam while in captivity and changed his name from Peter to Abdul-Rahman.
Kassig's parents, Ed and Paula Kassig, issued a statement via Facebook before the confirmation of the killing of their "treasured son."
They added: "The family respectfully asks that the news media avoid playing into the hostage takers' hands and refrain from publishing or broadcasting photographs or video distributed by the hostage takers. We prefer 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."
A volunteer for Libyan and then Syrian humanitarian efforts, Kassig's brother had previously urged his captors to "please show mercy" because "we need him, the world needs him."
A previous video -- released on October 3, purportedly by IS -- showed an IS masked militant threatening a man identified as Kassig.
Following the release of that video, the National Security Council had said it was using all available means to secure his release.
Speaking at the end of a G20 summit his country hosted, Australian Prime Minister Tony Abbott called IS a "death cult" and said there has been "a gruesome catalogue of beheadings, of crucifixions, of mass executions, of sexual slavery since [it] started to consolidate its hold in eastern Syria and northern Iraq."
British Prime Minister David Cameron said he was "horrified" by the video.
"I'm horrified by the cold-blooded murder of Abdul-Rahman Kassig. [IS] have again shown their depravity. My thoughts are with his family," Cameron said via Twitter.
French Prime Minister Manuel Valls condemned the apparent executions of the U.S. aid worker and Syrian soldiers as a "barbaric act" that would only boost France's determination to take on IS militants.
IS previously released videos showing the beheadings of two U.S. journalists -- James Foley and Steven Sotloff -- and of British nationals David Haines and Alan Henning.
The group has executed scores of Iraqi and Syrian security personnel and carried out beheadings on camera.
The militant group has also killed hundreds of Iraqi and Syrian tribesmen who opposed it, attacked religious and ethnic minorities, and sold women as slaves.
The U.S. military's top general meanwhile said on November 16 that U.S. forces have begun advising Iraqi troops at an airbase in the western Anbar province, much of which is controlled by IS.
The Obama administration said on November 7 that it would send up to 1,500 more U.S. troops to Iraq, widening its advising mission and initiating training of Baghdad's forces.
General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, told Reuters in connection with his surprise visit to Iraq that a small group of advisers had already established themselves in a preliminary fashion at Ain al-Asad air base.
He said they "are already working with the [Iraqi army's] seventh division to help them plan and help them understand the threat, to advise them on how to consolidate their forces."
Dempsey said the U.S. forces would also eventually start training the seventh division.