UK Prime Minister David Cameron late Saturday appeared to confirm the execution of British aid worker David Haines by the extremist group Islamic State (also known as ISIS or ISIL), calling it an "act of pure evil" and vowing justice.
"The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude," Cameron said in a statement.
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
ISIS released a video late Saturday evening claiming to show the beheading of Haines, the third Westerner the group has brutally murdered over the past month. The other two executions, of American journalists James Foley and Steven Sotloff, led US President Barack Obama to expand a military campaign against the group.
Haines' beheading appeared to come at the hands of the militant known as "Jihadi John," who speaks English in a British accent and was featured in the two previous execution videos.
The British Foreign Office has said it is "working urgently" to verify the video. It said that if authenticated, it would prove to be another "disgusting murder" by the group.
Haines was working in Syria for the French aid humanitarian aid agency ACTED. He was kidnapped after only three days in Syria, and was reportedly later sold to ISIS.
After ISIS revealed it was holding Haines hostage in the video showing the execution of Sotloff, ACTED said it "strongly condemns the violence and threats against David. A man’s life should never be threatened on account of his humanitarian commitment."