Islamic State Beheads British Hostage David Haines

The Islamic State released a video Saturday, showing the execution of British hostage David Haines.

Haines became the third western hostage killed by the Islamic State recently after the group executed U.S. journalists Steven Sotloff and James Foley recently.

British Prime Minister David Cameron condemned the killing of Haines and said that he will do everything in his power to hunt down the murderers.

"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil. My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal.

"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," he said, reports The Guardian.

In the video titled, "A message to the Allies of America", a masked man is shown carrying out the killing of Haines. Haines also was forced to read out a statement addressed to British Prime Minister Cameron.

"You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor, Tony Blair, did, following a trend amongst our British prime ministers who can't find the courage to say no to the Americans.

"Unfortunately, it is we, the British public that will in the end pay the price for our Parliament's selfish decisions," Haines said in the video under duress.

The Islamic State militant then says in the video that Haines has to pay the price for Cameron's decision to help the Peshmarga fighters in fighting the Islamic State. He adds that Britain's association with America will speed up its destruction. The masked militant further says that Cameron was playing the role of a lapdog and that will drag the British people in to a violent and  unwinnable war.

At the end of the video, another hostage, whom the militant identified as Alan Henning, a British citizen, is shown. The militant said that he also would be killed if Cameron continues to support the fight against Islamic state.

The British government had kept the kidnapping of Hailey secret, fearing the risk to his life. But his kidnapping became public when the militants posted the video showing the execution of Steven Sotloff and identified Haines as a hostage and the next victim, reports Associated Press.

On Friday, Haines' family had appealed to his captors to respond to their messages.

In a short statement released through the Britain foreign commonwealth office, the family told the militants that they wanted them to contact the family, reports AP.

Haines 44, was kidnapped near a refugee camp in northern Syria in March this year. He  worked for the French humanitarian agency ACTED.