British Prime Minister David Cameron said on Sunday he was "horrified" after the Islamic State jihadist group claimed to have executed US aid worker Peter Kassig, known as Abdul-Rahman after converting to Islam.
"I'm horrified by the cold-blooded murder of Abdul-Rahman Kassig," Cameron wrote on Twitter. "ISIL have again shown their depravity. My thoughts are with his family."
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The IS video, which is being examined by US intelligence officials and Britain's Foreign Office, also showed the gruesome simultaneous beheadings of at least 18 men described as Syrian military personnel, the latest in a series of mass executions carried out by IS militants.
Cameron called Kassig "a selfless humanitarian worker who had gone to the region to help" and said that the assumed killing "underlines the deplorable depths to which these terrorists are prepared to go".
The British government meanwhile has said it is looking into a newspaper report that "Jihadi John", a British-accented man who appeared in previous videos showing the killings of two US journalists and two British aid workers, had been injured in a US-led air strike last week.