Islamic State executes UK aid worker

The Islamic State group released a video on Saturday showing the beheading of British aid worker David Haines, who was abducted in Syria last year. The slaying, which British Prime Minister David Cameron has condemned as "an act of pure evil," follows the recent beheadings of two American journalists, James Foley and Steven Sotloff.

Cameron confirmed Haines' death in a statement posted late Saturday on his official Twitter account after the British Foreign Office had said earlier that it was "working urgently to verify the video."

In the statement, Cameron pledged to do "everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

The White House issued a similar statement shortly afterwards, characterizing Haines’ murder as “barbaric” and vowing to “destroy” Islamic State.

“We will work with the United Kingdom 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,” the statement said

Haines, a 44-year-old father of two from Perth in Scotland, was kidnapped last year while working for the French agency ACTED.

The video, titled "A Message to the Allies of America," opened with Cameron talking about working with the Iraqi government and allied Kurdish peshmerga forces to defeat Islamic State.

"This British man has to pay the price for your promise, Cameron, to arm the peshmerga against the Islamic State," said a masked man dressed in black with a British accent, standing over Haines, who was shown kneeling and wearing an orange jumpsuit.

Haines also spoke, saying Cameron was responsible for his execution.

"You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor, Tony Blair, did," he said.

"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."

Foley and Sotloff made similar speeches to President Barack Obama which have been dismissed as scripted by Islamic State and delivered under duress.

The video then showed the beheading of Haines.

At the end of the video, another yet-unidentified hostage was shown and the masked man said he would be killed if Cameron continues to support the fight against Islamic State.

The executioner appeared to be the same British-accented man who appeared in videos with Foley and Sotloff. Like the other videos, the latest was filmed in a desert and the captives wore orange jumpsuits.

The United States resumed airstrikes in Iraq in August for the first time since the withdrawal of the final U.S. troops from the country in 2011.

The raids followed major gains by Islamic State, which has declared an Islamic caliphate in areas it controls in Syria and Iraq.

Obama is now calling for a coalition of Western and Middle Eastern countries to fight Islamic State and has said the U.S. intends to bomb Islamic State positions in Syria.

Britain has delivered humanitarian aid, carried out surveillance, given weapons to Kurds and promised training in Iraq. On military action, Britain supports U.S. airstrikes and Cameron has repeatedly said Britain itself has ruled nothing out except combat troops on the ground.

Haines' family appealed earlier on Saturday to his captors to respond to their messages.

"We are the family of David Haines," relatives said in a statement released by Britain's Foreign 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."

Paris-based ACTED has previously said Haines had been engaged in humanitarian work since 1999, helping victims of conflicts in the Balkans, Africa and the Middle East and that he was taken hostage in March 2013 in Syria.

Al Jazeera and wire services