IS murder of British hostage an 'act of pure evil': Cameron

The beheading of a British hostage by Islamic State militants was an "act of pure evil," Prime Minister David Cameron said Sunday, vowing Britain will do all in its power to bring the killers to justice.

British Prime Minister David Cameron leaves 10 Downing Street in central London on 3, September 2014, after chairing an emergency meeting following the execution of a second US journalist by Islamist fighters in Iraq and the threat that a British hostage will be next.

"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.