British Prime Minister David Cameron
British Prime Minister David Cameron

BEIJING, Sept. 14 (Xinhua/Sun) -- A video footage released by the Islamic State (IS) militants on Saturday purportedly showed the beheading of British aid worker David Haines, making him the third Western captive to be killed by the group in recent weeks.

The IS militants said the execution of Haines, a 44-year-old Scottish-born father of two, was a "message to the allies of America."

Although the footage was yet to be verified, it was believed consistent with similar images in execution of two American journalists, James Foley and Steven Sotloff several weeks ago.

The execution of Haines was in retaliation for British Prime Minister David Cameron's promise to join a U.S.-led coalition to fight the IS, said a masked IS militant in the video.

The masked man with the same British accent of the previous videos warned Cameron of his recent rhetoric on efforts to defeat the group, saying that he has to "pay the price" for his promise.

At the end of the video, the militant, who appeared to be the same killer in the previous videos, threatened to kill another British hostage named Alan Henning, if Cameron continued to support the fight against the IS.

In a statement released by his Downing Street office in the early hours on Sunday, Cameron called Haines' beheading an "act of pure evil."

"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil," he said in the statement.

He vowed everything in Britain's power to "hunt down these murderers and ensure they face justice, however long it takes."

The cruel killing came the same day that Haines' family appealed to his captors to contact them in a message delivered through the British Foreign and Commonwealth Office. "We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us," it said.

Haines worked for the Paris-based Agency for Technical Cooperation and Development, a non-governmental humanitarian agency.

He was abducted in March 2013 near a refugee camp in Atmeh, Syria, when arranging for the delivery of humanitarian aid to people in the camp.

U.S. President Barack Obama also strongly condemned the beheading of Haines on Saturday.

"The United States strongly condemns the barbaric murder of UK citizen David Haines," Obama said in a statement issued by the White House.

Obama said that the United States would "stand shoulder to shoulder tonight with our close friend and ally in grief and resolve."

He pledged to work with Britain and a broad coalition of nations from the region and around the world to "bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world."

Obama came out strong recently against the IS amid heightened pressure at home to deal seriously with the group as its militants threatened to "raise the flag of Allah in the White House."

"If you threaten America you will find no safe haven," Obama said in Wednesday's national television speech.

However, skepticism remained on how workable his plan would be because it fell short on details and rest mainly on a coalition with states and those fighting the IS on the ground.