ISIS video claims to show beheading of British hostage David Haines

Islamic State militants have released a video claiming to show the beheading of British aid worker David Haines, in what British Prime Minister David Cameron has described as "an act of pure evil."
The 44-year-old 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.
Prime Minister David Cameron described the apparent murder as "an act of pure evil" in a tweet from his official account. 
Sky News reports that Cameron is returning to Downing Street to chair a COBRA emergency response meeting.
The UK Foreign Office had said earlier in a statement that they were "working urgently to verify" the video.
"If true, this is another disgusting murder," the Foreign Office said in the 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".
Late Friday, the family of Haines issued a public plea urging his captors to contact them.