The Islamic State jihadist group claimed it executed British aid worker David Haines, in retaliation for British leader David Cameron entering a coalition with the United States against the militants.
This would be the third such execution in recent weeks, after two US journalists taken hostage in Syria were shown murdered.
The Islamist group released a video, available on the website of private terrorism monitoring group SITE, purportedly showing a masked militant beheading Haines.
David Haines had, as an aid worker, been active in conflict areas all over the world including Libya, South Sudan, Syria, and Croatia where he decided to settle with his Croatian second wife.
He was abducted by Islamic terrorists in Syria last year but his identity remained largely unknown until he featured at the end of the ISIS video showing the beheading of American Journalist Steven Sotloff.
The British Foreign Office said late Saturday that it was “working urgently to verify” the video.
“If true, this is another disgusting murder,” the Foreign Office said in a statement.
“We are offering the family every support possible. They ask to be left alone at this time.”
Islamic State militants have beheaded two American journalists, James Foley and Steven Sotloff, as well as Kurdish and Lebanese fighters, and posted video evidence online. At the end of the last video showing the beheading of American journalist Steven Sotloff, the Islamic State group threatened to kill Haines next and briefly showed him on camera.
In the video posted Sunday, the group threatened to kill another Briton. Both British men were dressed in orange jumpsuits against an arid Syrian landscape, similar to that seen in the Foley and Sotloff videos.
The SITE Intelligence Group, a U.S. terrorism watchdog, reported the video, which was also posted online by users associated with the Islamic State group.
The video was entitled “A Message to the Allies of America.” Haines’ purported killer, who appeared to be the same man speaking with a British accent as in the previous videos, tells the British government that its alliance with the U.S. will only “accelerate your destruction” and will drag the British people into “another bloody and unwinnable war”.
Haines was abducted in Syria in 2013 while working for an international aid agency.
The British government had managed to keep his kidnapping secret out of concern for his safety until the most recent video identified him as a captive.
Late Friday, the family of Haines issued a public plea urging his captors to contact them.
In a short statement released through Britain’s Foreign and Commonwealth Office, the family said: “We are asking those holding David to make contact with us.”
AFP and AP contributed to this report