'IS' claims US hostage Kassig executed

United States intelligence officials were examining the video, released to social networks on Sunday that displayed a black-clad militant standing with a severed head he claims is that of the US aid worker.

"The intelligence community is working as quickly as possible to determine its authenticity," National Security Council spokeswoman Bernadette Meehan said in a statement.

She added: "If 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."

Kassig, 26, was captured last year while helping provide medical aid to Syrians. His friends say he converted to Islam in captivity and changed his first name from Peter to Abdul-Rahman.

If the claim is true, he would be the fifth hostage from the US or UK to be executed on video by IS militants.

Mass execution of Syrian soldiers

The video also showed what appeared to be the simultaneous mass beheadings of at least 18 men, described by 'Islamic State' as Syrian military pilots and officers loyal to President Bashar al-Assad.

The militants warn that United States soldiers would meet a similar fate and that the IS fighters were "eagerly" waiting for them.

It is the latest in a series of atrocities, including mass executions, carried out by IS, which has claimed large areas of territory in both northern Syria and Iraq.

Sunday's video was released as IS suffered losses in battles in Iraq, which had been backed by US-led air strikes.

'Cold-blooded murder'

British Prime Minister David Cameron said on Sunday he was "horrified" by the purported killing of Kassig.

The 'Islamic State' video made no mention of Kassig's aid work, mentioning only his time as a soldier in Iraq.

Kassig's family said he had helped to train 150 civilians to provide medical aid to people in Syria and the aid group he founded had given food, cooking supplies, medicines and clothing to displaced Syrians.

Kassig had written to his parents, saying: "If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need."

se/glb (Reuters, AP, AFP, dpa)