**EDITORS PLEASE NOTE: THIS EDIT CONTAINS GRAPHIC / REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY OF THIS VIDEO**
Islamic State militants fighting in Iraq and Syria released video Saturday purportedly showing the beheading of British aid worker David Haines.
Reuters could not immediately verify the footage.
The images are consistent with that of the filmed executions of two American journalists, James Foley and Steven Sotloff.
British Prime Minister David Cameron said Saturday the murder was an act of pure evil and vowed to bring his killers to justice however long it takes.
Haines, a 44-year-old father of two from Perth in Scotland, was kidnapped last year while working for a French AID agency.