The Islamic State jihadist group on Sunday claimed to have executed US aid worker Peter Kassig as a warning to the United States.

The extremist group released a video showing the simultaneous beheadings of at least 15 men described as Syrian military personnel. In the highly choreographed sequence, jihadists march 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.