British Prime Minister David Cameron vowed on Sunday that Britain would do all it could to catch the killers of a British aid worker beheaded by the Islamic State, which he condemned as an "act of pure evil".

In its third beheading of a Western hostage in less than a month, IS released a video late Saturday showing the execution of Briton David Haines and threatening the life of another British captive.

Cameron led a meeting of the government's emergency Cobra committee early Sunday in response to the online video, which he said showed "a despicable and appalling murder of an innocent aid worker" and "an act of pure evil."