British aid worker David Haines is the third western hostage to be killed by Islamic State (IS) militants.
His death, apparently filmed in footage uploaded by IS online, follows those of US journalists James Foley and Steven Sotloff.
Their deaths prompted US president Barack Obama to announce a detailed plan to defeat IS in both Iraq and Syria, where the group has gained a foothold amid conflict and instability in the Middle Eastern countries.
Mr Sotloff was seized in Syria in August 2013 and had not been seen until he appeared in the video of Mr Foley's killing released by IS on August 20, while there had been a failed attempt to rescue Mr Foley, who was seized in Syria in 2012.
As in both previous releases, the video was uploaded to social media several weeks apart and feature a masked man standing next to the victims in a desert.
The victims then address the camera before the militant speaks.
The masked killer in the latest video is thought to be the same jihadist with a London accent who carried out the on-screen murder of both US reporters.
The murder of Mr Foley sparked a scramble by British intelligence agencies to identify the killer, who has been dubbed ''jihadi John''.
The rise of the Islamic State of Iraq and the Levant (Isis) has been aided by the continuing failure of the US Government to investigate the role of Saudi Arabia in the 9/11 attacks and its support of jihadi movements such as al-Qaeda in the years since, says former Senator Bob Graham, the co-chairman of the official inquiry into 9/11.