Video shows British aid worker beheaded

BEIRUT -- Islamic State extremists have released a video showing the beheading of British aid worker David Haines, who was abducted in Syria last year, and British Prime Minister David Cameron is condemning his slaying as "an act of pure evil."

Cameron confirmed Haines' death in a statement posted late Saturday on his official Twitter account after the British Foreign Office had said earlier that it was "working urgently to verify the video."

Cameron has returned to his residence at Downing Street and is expected to chair a meeting of the government's emergency response team on Sunday.

Cameron vows to do "everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

Islamic State militants earlier beheaded two American journalists, James Foley and Steven Sotloff, as well as Kurdish and Lebanese soldiers, and posted video evidence online.

"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."

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 also was posted online by users associated with the Islamic State group.

The video was titled "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 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."