Media playback is unsupported on your device Media caption David Haines was being held by Islamic State militants, as Caroline Hawley reports 
 
 The murder of British hostage David Haines was an "act of pure evil", David Cameron has said after the release of a video of his beheading. 'Despicable and appalling' 
 
 In a statement released by the Foreign Office, Mike Haines said his brother, a father of two, "was and is loved by all his family". "David was most alive and enthusiastic in his humanitarian roles," said Mike Haines, David's brother, in a statement released by the U.K. Foreign Office early Sunday local time. His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair," he said. Image copyright Sky Image caption Downing Street said the prime minister had returned to No 10 following the release of the video 
 
 Mr Cameron, who chaired an emergency Cobra committee meeting on Sunday morning, said the murder of an innocent aid worker was "despicable and appalling". It is an act of pure evil," Mr. Cameron said in a statement published shortly after midnight in London. "My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal," he said. Britain will do everything in its power to hunt down the killers of Mr. Haines and ensure they face justice, "however long it takes," Mr. Cameron said. 'Grief and resolve' 
 
 The Foreign Office is working to verify the video, which was released on Saturday night. The militant, who appears to have a British accent, is then recorded as saying: "This British man has to pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State." Last week, the British began delivering a gift of heavy machine guns and nearly half a million rounds of ammunition to Kurdish regional forces battling Islamic State insurgents, the first time it has directly supplied weapons to the Peshmerga in this conflict. International condemnation 
 
 Mr Haines's death has been condemned by international political leaders, including Australian Prime Minister Tony Abbott, French President Francois Hollande, Labour leader Ed Miliband and Scottish First Minister Alex Salmond. The killing comes only days after U.K. Prime Minister David Cameron joined U.S. President Barack Obama in declaring a strategy for attacking the Islamist group and embarking upon a regionwide effort to recruit support for a plan to reverse the gains of Islamic State, also called ISIS or ISIL. The statement said that Mr. Cameron had followed "a trend amongst our British prime ministers who can't find the courage to say no to the Americans," and that Mr. Cameron was responsible for Mr. Haines's killing. Mr Alex Salmond said a resilience meeting - the Scottish government's version of the UK's Cobra security meeting - would be held on Sunday morning to discuss issues such as the privacy of Mr Haines's family. The leader of the Better Together campaign in Scotland, Alistair Darling, said Mr Haines's death "inexcusable, a barbaric act" that would "strengthen the resolve of the international community". Image copyright EPA Image caption The family of US journalist Steven Sotloff said he had given his life to reporting from war zones 
 
 Mr Haines was taken hostage in the village of Atmeh, in the Idlib province of Syria, in March 2013. Mr. Haines's family had tried to make contact with his captors several times, including a direct appeal on Saturday. Photo: Getty Images 
 
 The White House and Pentagon said Friday that the U.S. is "at war" with Islamic State, which has grabbed tracts of territory in Iraq and Syria. They threatened to kill Mr Haines during a video posted online showing the killing of US journalist Steven Sotloff earlier this month.
– David Cameron emerged from an emergency meeting this morning with harsh words for Islamic State militants in the wake of yesterday's video that purports to show the beheading of British aid worker David Haines: Calling the execution—which hasn't been officially verified, though a UK Foreign Office rep tells the Wall Street Journal that "all signs are that the video is genuine"—"an act of pure evil," the BBC reports that Cameron vowed "we will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes." In the video, Haines singled out Cameron thusly: "I would like to declare that I hold you, David Cameron, entirely responsible for my execution." ISIS militants threatened a second British hostage, Alan Henning, with the same fate. The White House issued a statement last night, as per the Journal, saying that President Obama "strongly condemns the barbaric murder of UK citizen David Haines," and that "Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom." Haines' family, which the AP notes had asked ISIS to contact them, said in a statement that Haines' "joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair."