Cameron described the apparent beheading as "an act of pure evil" and promised to hunt down the killers.

"This is a despicable and appalling murder of an innocent aid worker," said Cameron in a statement released by Downing Street. "My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal."

Cameron vowed that the "Islamic State" (IS) members responsible would be caught "however long it takes" - a sentiment he reiterated via Twitter.

Britain's foreign ministry had earlier said it was working to confirm the authenticity of the video - believed to have been filmed in Syria - that claimed to show the beheading.

IS on Saturday claimed the beheading in what would be the third such execution in recent weeks.

Video's 'message to allies'

The video, "A Message to the Allies of America," blames British Prime Minister David Cameron for entering a coalition with the United States, which has said it is at "war" with the IS and has launched air strikes against them in Iraq.

"You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor Tony Blair did, following a trend amongst our British prime ministers who can't find the courage to say no to the Americans," said Haines in the video.

Haines, a father of two from Perth in Scotland, was kidnapped in Syria in March 2013 while working for the Agency for Technical Cooperation and Development (ACTED), a French relief charity. He had previously been involved in humanitarian work in other parts of the Middle East as well as the Balkans and Africa.

Haines' family had issued a plea on Saturday urging his apparent captors to contact them. "We have sent messages to you, to which we have not received a reply," the family said in a statement released by the British Foreign Office.

Steven Sotloff and fellow US journalist James Foley were also kidnapped in Syria. IS released a video claiming it had executed Foley on August 19, with the video of Sotloff's death coming two weeks later on September 2.

rc/crh (AP, AFP, Reuters)