Islamic State claims it has beheaded US hostage Kassig
Video released on Sunday allegedly shows Kassig's severed head; Kassig captured last year and converted to Islam in captivity.
The nearly 16-minute video uploaded to social networks on Sunday did not show the beheading but showed a masked man standing with a decapitated head covered in blood lying at his feet. Speaking in English in a British accent, the man says: "This is Peter Edward Kassig, a US citizen."
It was not immediately possible to confirm that Kassig was pictured in the footage, which appeared on a jihadist website and on Twitter feeds used by Islamic State. Family representatives were not immediately available for comment.
Kassig's parents have said through a spokesperson their son was taken captive on his way to the eastern Syrian city of Deir al-Zor on October 1, 2013.
A former soldier, Kassig was doing humanitarian work through Special Emergency Response and Assistance, an organization he founded in 2012 to help refugees from Syria, the family has said.
If confirmed, Kassig's beheading would be the fifth such killing of a Westerner by Islamic State, following the deaths of two US journalists and two British aid workers.
The video marks the first time the Islamic State has clearly stated where the beheading took place, in the village of Dabek on the outskirts of Aleppo.
In an apparent reference to testimony about Kassig by former fellow captives, the masked man says: "Peter, who fought against the Muslims in Iraq while serving as a soldier under the American army, doesn't have much to say. His previous cellmates have already spoken on his behalf."
The video also showed what appeared to be the mass beheading of several captured Syrian pilots and officers loyal to President Bashar Assad.
"To Obama, the dog of Rome, today we are slaughtering the soldiers of Bashar and tomorrow we will be slaughtering your soldiers," the masked militant said.
British Prime Minister David Cameron said he was horrified by the "cold-blooded murder".
Both Britain's Foreign Office and the US intelligence community are working to determine the authenticity of the video.
US National Security Council spokeswoman Bernadette Meehan said that if the video is authentic, the White House would be "appalled by the brutal murder of an innocent American."
She added the White House expresses its deepest condolences to Kassig's family and friends.
Reuters and The Associated Press contributed to this report.
