A still image taken from a purported Islamic State video released Sept 13, 2014 of British captive David Haines before he is beheaded. The images were consistent with that of the filmed executions of two American journalists, James Foley and Steven Sotloff, in the past month. Haines, a 44-year-old father of two from Perth in Scotland, was kidnapped last year. [Photo/Agencies]

BEIRUT - Islamic State extremists have released a video showing the beheading of British aid worker David Haines, who was abducted in Syria last year, and British Prime Minister David Cameron is condemning his slaying as "an act of pure evil."

Cameron confirmed Haines' death in a statement posted late Saturday on his official Twitter account after the British Foreign Office had said earlier that it was "working urgently to verify the video."

Cameron has returned to his residence at Downing Street and is expected to chair a meeting of the government's emergency response team on Sunday.

Cameron vows to do "everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

Islamic State militants earlier beheaded two American journalists, James Foley and Steven Sotloff, as well as Kurdish and Lebanese soldiers, and posted video evidence online.