London: Prime Minister David Cameron on Sunday called the murder of a British hostage by Islamic State militants an "act of pure evil" and vowed Britain would do all in its power to bring the killers to justice.
"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil," he said in a statement released by his Downing Street office.
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
Cameron will chair a meeting of the government`s emergency Cobra committee on Sunday in response to an online video purportedly showing a masked militant killing Haines.
The video appeared a day after the aid worker`s family on Saturday appealed to his captors to contact them.
Prime Minster Cameron tweeted that his "heart goes out to his family who have shown extraordinary courage and fortitude."
Britain`s Foreign Office said they were "working as quickly as we can to try and verify" the video.
Two US journalists have been murdered in similar circumstances in recent weeks.