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. 
 
 The 44-year-old aid worker was seized in Syria in 2013. He was being held by Islamic State militants who have already killed two US captives. 
 
 The latest video also includes a threat to kill a second British hostage. 
 
 The PM vowed to do everything possible to find the killers. Mr Haines's family said he would be "missed terribly". 
 
 Born in Holderness, East Yorkshire, Mr Haines went to school in Perth and had been living in Croatia with his second wife, who is Croatian, and their four-year-old daughter. His parents live in Ayr. 
 
 '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. 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. 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," the prime minister added. 
 
 'Grief and resolve' 
 
 The Foreign Office is working to verify the video, which was released on Saturday night. It begins with a clip of Mr Cameron and then features a man who appears to be Mr Haines dressed in orange overalls, kneeling in front of a masked man holding a knife. 
 
 The victim says: "My name is David Cawthorne Haines. I would like to declare that I hold you, David Cameron, entirely responsible for my execution." 
 
 He says Mr Cameron had entered into a coalition with the US against the Islamic State "just as your predecessor Tony Blair did". 
 
 "Unfortunately it is we the British public that in the end will pay the price for our parliament's selfish decisions," he said. 
 
 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." 
 
 Image copyright Unknown Image caption In the video a masked militant is seen standing beside a man who appears to be David Haines 
 
 Islamic State is now in control of large parts of northern Iraq and Syria, and the CIA estimates that the group could have as many as 30,000 fighters in the region. 
 
 The UK has donated heavy machine guns and ammunition to authorities in Iraq to help fight IS militants, the Ministry of Defence has previously said. 
 
 Kurdish forces, known as the Peshmerga, have been involved in heavy fighting with IS. 
 
 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. 
 
 US President Barack Obama said the US would work with the UK and other countries to "bring the perpetrators of this outrageous act to justice". 
 
 Deputy Prime Minister Nick Clegg said the government "will not rest" until Mr Haines's killers "face justice". 
 
 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". 
 
 Justice Secretary Chris Grayling said he would like to see the militant in the video "face justice in a British court - he's a British citizen and has been boasting of that". 
 
 Analysis: Jim Muir, BBC News 
 
 The beheadings of David Haines, and the two American journalists before him, are believed to have been filmed in the desert near Raqqa, a provincial capital in north-east Syria which has become the unofficial capital of the self-styled Islamic State. 
 
 Far from being just a shadowy terror group, IS controls a large swathe of territory in the north and east of Syria, linked since June to almost all the mainly Sunni parts of Iraq. 
 
 It controls a population of around 5 or 6 million people, including Iraq's second city, Mosul. 
 
 In northern Iraq, the Kurds, helped by US air strikes, are slowly regaining the ground they lost to IS when it suddenly turned on them last month. 
 
 That's the template the Americans and their allies want to apply to other parts of Iraq, and Syria, but the situation in both countries is complicated by the absence of clear-cut, cohesive local forces on the ground to work with. 
 
 Elsewhere, religious leaders have also reacted to the killing, including the Archbishop of Canterbury, Justin Welby, who posted a call on Twitter for prayers for the family of Mr Haines, who was "evilly killed". 
 
 Shuja Shafi of the Muslim Council of Britain said: "These extremists wish to draw attention and recruits to their cause by sowing division and fear between people here in Britain. Let us deny them that luxury." 
 
 'Criminals and villains' 
 
 Colonel Richard Kemp, former commander of British forces in Afghanistan, said it was vital that Britain took some "serious action" against IS. 
 
 "The key issue here is that we cannot rule out the use of large-scale ground forces. I don't mean the kind of thing that's happening now. I mean large-scale intervention forces," he told BBC Radio 5 live. 
 
 Failure to do so would mean Britain would "just accept the fact that the Islamic State will continue to expand, continue to decapitate our citizens, continue to pose a threat to our country and countries in the region". 
 
 But former military intelligence officer Frank Ledwidge said Britain "must be sober about this, and understand our national interests and equate what we do with those interests". 
 
 He said IS's objective now is "to hold what it's got, and there's no better way of doing that than recruiting to it the millions of people in the area who'd be quite happy to fight yet another Western intervention in the area". 
 
 Paul Rogers, professor of peace studies at Bradford University, said the UK's natural reaction would be to "go in hard" against IS, but that this presented a problem, because the extremists wanted to be attacked so as to "present itself as fighting the far enemy". 
 
 Mr Rogers also said the "war" in the region was "at a much higher level than we suppose", with more than 2,000 sorties by the US in the last few weeks. 
 
 The West was now moving into "what is essentially a third Iraq war" and that this time it would extend into Syria, he said. 
 
 Image copyright AP Image caption US journalist James Foley was killed by militants last month - his parents called him a "martyr for freedom". 
 
 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. 
 
 He had been helping French agency Acted deliver humanitarian aid, having previously helped local people in Libya and South Sudan. 
 
 The release of the video came hours after his family made a direct appeal to IS to contact them. 
 
 IS - also known as Isis or Isil - has seized large swathes of territory in Iraq and Syria and declared a new caliphate - or Islamic state. 
 
 Militants from the extremist group have killed two US hostages in recent weeks, posting video evidence on the internet. 
 
 They threatened to kill Mr Haines during a video posted online showing the killing of US journalist Steven Sotloff earlier this month. 
 
 The extremist group also killed fellow US journalist James Foley last month. ||||| The following statement was issued Saturday by the White House’s press secretary: 
 
 The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL. Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom. The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve. 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. 
 
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 ||||| Militants from the Islamic State released a video which purports to show the beheading of British aid worker, David Haines. Photo: AP 
 
 BAGHDAD—The Islamic State militant group released a video late Saturday that it said showed the beheading of David Haines, a British aid worker. 
 
 Islamic State militants had previously released videos showing the beheadings of American journalists James Foley and Steven Sotloff. 
 
 The video released Saturday showed what a masked person said was Mr. Haines's beheading before proposing that Alan Henning, another Briton, might face the same fate if British forces didn't stop their aggression against the militant group. 
 
 A spokesman for the U.K.'s Foreign Office said Sunday that "all signs are that the video is genuine." 
 
 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. 
 
 U.K. Prime Minister David Cameron pledged to destroy the militant Islamic group responsible for the beheading of British aid worker David Haines. 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. President Obama has authorized the start of U.S. airstrikes in Syria and expanded a monthlong bombing campaign in Iraq to degrade and ultimately destroy Islamic State militants. 
 
 "The United States strongly condemns the barbaric murder of U.K. citizen David Haines by the terrorist group ISIL," Mr. Obama said in a statement on Saturday evening. "Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom," he said. 
 
 Mr. Obama said that the U.S. will work with the U.K. and a broad coalition of nations "to bring the perpetrators of this outrageous act to justice." 
 
 In a statement in the video apparently dictated by his captors, Mr. Haines faulted Mr. Cameron for following Mr. Obama into the Middle East. 
 
 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. 
 
 Messrs. Foley and Sotloff read similar, apparently forced messages blaming Mr. Obama for his actions. 
 
 The U.K.'s Foreign Office said it was verifying the authenticity of the video. 
 
 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. 
 
 "This is a despicable and appalling murder of an innocent aid worker. 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. 
 
 In the same statement, a government spokesperson said the prime minister would chair a meeting of the government's COBR ad hoc crisis committee on Sunday morning. 
 
 Ed Miliband, the leader of the opposition Labour Party, said he was sickened by the killing and offered his deepest condolences to Mr. Haines's family. 
 
 "Acts like this will not weaken but strengthen the resolve of Britain and the international community to defeat ISIL and their ideology," he said in a statement. 
 
 Mr. Haines, 44, worked as a humanitarian since 1999, helping victims of conflict in the Balkans, Africa and the Middle East, according to a Sept. 5 statement from ACTED, a Paris-based nongovernmental organization. When he was taken hostage in March 2013, Mr. Haines was in Syria as part of ACTED's humanitarian effort in the war-torn country. 
 
 Mr. Haines's family had tried to make contact with his captors several times, including a direct appeal on Saturday. 
 
 "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." 
 
 Mike Haines described David as "a good brother" and the father of two daughters. "David served with the U.N. in the Balkans, helping people in real need...He helped whoever needed help, regardless of race, creed or religion," he said. 
 
 The video released by Islamic State on Saturday showed what appeared to be similar scenes to those in the previous two videos that showed the killings of Messrs. Foley and Sotloff. 
 
 In the video, a masked militant, dressed entirely in black, delivers a speech against the backdrop of a barren desert landscape. The militant appears to have a British accent. The other two videos also showed militants apparently speaking with a British accent. 
 
 The videos have fueled concerns about Britons being drawn into the conflict in Syria and Iraq and the risk they might return home radicalized, battle-hardened and intent on carrying out attacks on U.K. soil. Earlier this month, Mr. Cameron announced plans to strengthen powers to counter the security threat posed by Islamist extremists at home and those traveling in and out of Britain, including legislation to enable police to seize passports at the border. 
 
 The U.K. government hasn't ruled out airstrikes against Islamic State militants in Iraq or Syria, Mr. Cameron's spokesman said on Thursday. Up to now, it has pursued a strategy of trying to put pressure on Islamic State by supporting a unity government in Iraq, working to make sure Kurdish regional forces have the arms and supplies they need to fight the extremists, and providing humanitarian aid. The government has ruled out committing troops to fight in Iraq. 
 
 The British air force has already been used for surveillance flights and to deliver humanitarian supplies and arms to Kurdish regional forces in northern Iraq. 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. 
 
 Write to Matt Bradley at matt.bradley@wsj.com and Nick Winning at nick.winning@wsj.com
– 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."