Cameron returned to his residence at 10 Downing Street shortly after midnight and is expected to chair a meeting of the government's emergency response committee early Sunday.

The video emerged a day after Haines' family issued a public plea late Friday urging his captors to contact them.

The Foreign Office said it was offering Haines' family "every support possible. They ask to be left alone at this time."

Islamic State militants recently beheaded two American journalists, James Foley and Steven Sotloff, and posted the videos online after the U.S. began launching airstrikes and humanitarian missions in August to aid waning Iraqi and Kurdish security forces in northern Iraq. The Islamic State group has also put out videos showing the beheading of Kurdish and Lebanese soldiers and the mass shooting of dozens of captured Syrian soldiers.

At the end of the video showing the beheading of Sotloff, the Islamic State group threatened to kill Haines next and briefly showed him on camera.

President Barack Obama issued a statement strongly condemning "the barbaric murder" of Haines by the Islamic State terrorist group.

"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," Obama said.

On Wednesday, Obama gave a nationally televised speech outlining a potential military campaign to "degrade and ultimately destroy" the Islamic State militants that is likely to include training and equipping moderate Syrian rebels and Iraqi forces, providing intelligence, and expanding airstrikes against extremists in Iraq and potentially into Syria. Britain was among the initial group of countries joining the U.S.-led coalition to root out the Islamic State extremists.

In the video showing Haines' slaying, the group threatened to kill another British captive. Both British men were dressed in orange jumpsuits against an arid Syrian landscape, similar to that seen in the Foley and Sotloff videos.

The video was entitled "A Message to the Allies of America." Haines' killer, who appeared to be the same masked man speaking with a British accent and brandishing a knife as in the previous videos, tells the British government that its alliance with the U.S. will only "accelerate your destruction" and will drag the British people into "another bloody and unwinnable war".The SITE Intelligence Group, a U.S. terrorism watchdog, reported the video, which was also posted online by users associated with the Islamic State group.

In the video, according to a transcript provided by SITE, Haines says that he holds Cameron "entirely responsible for my execution" because Britain has "voluntarily entered into a coalition with the United States against the Islamic state."

His killer says in the video that Haines was paying the price for Britain's decision to supply weapons to Kurdish Peshmerga fighters, who are battling the Islamic State group in northern Iraq. He references U.S. airstrikes against Islamic State fighters near the Haditha Dam in western Iraq, which the U.S. began targeting on Sept. 7 -- indicating that Haines' killing took place sometime during the past week.

The British government had managed to keep Haines' kidnapping secret out of concern for his safety until the Sotloff beheading video identified him as a captive and the next victim.

Late Friday, the family of Haines issued a public plea urging his captors to contact them.

In a short statement released through Britain's Foreign and Commonwealth Office, the family said: "We are asking those holding David to make contact with us."

Haines, 44, was abducted near a refugee camp in northern Syria in March 2013 while working for the French international aid agency ACTED which was helping thousands of Syrians displaced by the fighting between Syrian President Bashar Assad's forces and rebel groups, including the Islamic State, seeking to oust him. ACTED said in a statement earlier this month that Haines had worked with as an international aid worker since 1999, helping victims of conflict in the Balkans, Africa and the Middle East. Before heading to Syria, ACTED said Haines had worked in 2012 as a "non-partisan unarmed civilian peace facilitator" in South Sudan.

Michael Rubin, a resident scholar at the conservative American Enterprise Institute, said the Islamic State group releases such videos because it has concluded that neither the American nor the European public has the stomach for a protracted conflict in which civilians are tortured and killed.

If the slaying of Haines took place in the last few days, Rubin said, the timing suggests that U.S. and other forces have no idea where the extremists are keeping their captives and that, in turn, means a lack of intelligence about what goes on within the group itself."Each video that's released, of course, is a propaganda coup, if you will," Rubin, a Pentagon official during the George W. Bush administration, told The Associated Press in an interview Saturday night.

Mike Haines said in a statement issued by the Foreign Office that his brother was devoted to humanitarian work.

"David was most alive and enthusiastic in his humanitarian roles," Mike Haines said. "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 was and is loved by all his family and will be missed terribly."

British media have reported that Haines, who is married to a Croatian woman, was educated in Perth, Scotland, and spent 11 years serving in the British Royal Air Force before becoming an international aid worker. Britain's Press Association said Haines had a teenage daughter in Scotland from a previous marriage and a 4-year-old daughter from his present marriage.

視頻顯示了英國援助工作者殺害

ZEINA KARAM和Sylvia許，美聯社
2014年9月14日，下午2時26分TWN

伊斯蘭國家的極端分子公佈了一段視頻展示了英國援助工作者大衛·海恩斯，誰是敘利亞去年被綁架，和英國首相戴維·卡梅倫的斬首週六晚譴責殺害他為“純粹的邪惡行為。”

卡梅隆證實海恩斯逝世發表聲明後，英國外交部此前說，有人說“急著驗證視頻。” 海恩斯是第三西方人伊斯蘭國家集團，它抓住境內的敘利亞和伊拉克廣袤的大片斬殺在最近幾個星期。

“這是一個無辜的援助工作者的卑鄙和駭人聽聞的謀殺，它是純粹的邪惡行為，”卡梅倫說，補充說：“我的心臟出去大衛·海恩斯的家庭誰表現出非凡的勇氣和毅力這個磨難。 “

“我們將盡一切力量來追捕這些殺人犯，並確保他們面對司法，但需要多長時間，”卡梅倫說。

卡梅倫回到他的住所，在唐寧街10號的凌晨時分，預計主持政府緊急應對委員會會議日凌晨。

該視頻出現的前一天，海恩斯的家庭發出公開呼籲週五尾盤敦促他的綁架者與他們聯繫。

外交部說，這是海恩斯提供“家庭”每一個可能的支持，他們要求被單獨留在這個時候。“

伊斯蘭國家武裝分子最近斬殺兩名美國記者詹姆斯·福利和史蒂芬Sotloff，並張貼在網上的視頻後，美國就開始發射空襲和人道主義任務八月，以幫助伊拉克減弱和庫爾德安全部隊在伊拉克北部。伊斯蘭國集團還推出了視頻顯示的庫爾德人和黎巴嫩士兵斬首和幾十個拍攝的敘利亞士兵的傳射。

在顯示Sotloff的斬首視頻的最後，伊斯蘭國家組織威脅要殺死海恩斯旁邊，並簡要表明了他的相機。

美國總統奧巴馬發表聲明，強烈譴責海恩斯的“野蠻謀殺”伊斯蘭國家的恐怖組織。

“美國並肩站在今晚承擔與我們的親密朋友和盟友在悲痛和決心，我們將與英國，並從該地區和世界各國的廣泛的聯盟合作，將這種無恥行為的人繩之以法和降解，破壞我們國家的人來說，這威脅了地區和世界，“奧巴馬說。

週三，奧巴馬發表了全國電視講話概述了潛在的軍事活動，以“降低並最終消滅”伊斯蘭國武裝分子很可能包括訓練和裝備溫和敘利亞反政府武裝和伊拉克部隊提供情報，並擴大空襲反對伊拉克極端分子並有可能進入敘利亞。英國是個國家加入美國領導的聯軍剷除伊斯蘭國極端分子的初始群體中。

在顯示海恩斯'殺害的視頻，該集團威脅要殺死另一個英國俘虜。兩個英國男人們穿著抗乾旱敘利亞景觀，類似於出現在弗利和Sotloff視頻橙色囚服。

本網站情報小組，是美國反恐監視，報告視頻，這也是在網上與伊斯蘭國家組關聯的用戶發布。

該視頻的標題是“一個信息給美國的盟友。” 海恩斯'殺手，誰似乎是相同的蒙面男子說話帶有英國口音，並揮舞著刀在以前的影片，講述了英國政府，其與美國的聯盟不僅將“加快你的毀滅”，並會拖累英國民眾進入“另一個血腥和無法取勝的戰爭”。

在視頻中，根據現場提供的成績單，海恩斯說，他認為卡梅倫“完全負責我的執行力”，因為英國已“自願進入對伊斯蘭國家與美國的聯盟。”

他說，兇手在視頻中說海恩斯是付出了代價的，英國的決定提供武器給庫爾德準軍事部隊的戰士，誰是對抗伊斯蘭國集團在伊拉克北部。他引用了美國對空襲附近的哈迪塞大壩在伊拉克西部，而美國開始瞄準在9月7日伊斯蘭國戰機 - 這表明海恩斯'殺人在過去一周發生的某個時候。

英國政府已設法保持海恩斯'綁架的秘密，因為擔心他的安全，直到Sotloff斬首錄像確定他作為俘虜和下一個受害者。

上週五，海恩斯的家人發出公開呼籲，要求逮捕他的人與他們聯繫。

在通過英國外交和聯邦事務部發布的一份簡短聲明，對家人說：“我們要求那些拿著大衛作和我們聯繫。”

海恩斯，44歲，被綁架近在敘利亞北部的一個難民營，2013年3月，而工作的法國國際援助機構ACTED這是幫助成千上萬的敘利亞人敘利亞總統巴沙爾·阿薩德的部隊與反政府組織之間的戰鬥，包括伊斯蘭國家取代，尋求把他趕下台。在一份聲明本月初ACTED說，海恩斯曾與作為國際援助工作者自1999年以來的工作是幫助衝突受害者在巴爾幹地區，非洲和中東地區。在前往敘利亞，擔任海恩斯表示，曾在2012年作為一個在蘇丹南部“無黨派手無寸鐵的平民的和平推動者”的工作。

邁克爾·魯賓，居民學者在保守的美國企業研究所表示，伊斯蘭國家組織發布這樣的視頻，因為它的結論是，無論是美國還是歐洲公眾對胃的長期衝突中，平民被拷打和殺害。

“這是釋放，當然，每一個視頻，是一個宣傳妙招，如果你願意，”魯賓，喬治·W·布什執政期間，五角大樓官員告訴美聯社記者在採訪星期六晚上。

如果海恩斯的殺害，在過去幾天裡發生的，魯賓說，時間顯示，美國和其他部隊不知道那裡的極端分子保持他們的俘虜，而且，反過來，意味著缺乏智力有關所發生的內組本身。

邁克·海恩斯在由外交部發表聲明說，他的弟弟被用於人道主義工作。

“大衛是最活的熱情在他的人道主義的角色，”邁克海恩斯說。“他的喜悅和期待的工作，他去幹什麼敘利亞是為自己和家人這整個傷心的事，他也得到所有家人的喜愛，並會非常想念的最重要因素。”

英國媒體報導說，海恩斯，誰娶了一位克羅地亞的女子，曾就讀於珀斯，蘇格蘭，花了11年的英國皇家空軍服務成為國際援助工作者面前。英國報業協會表示，海恩斯有一個十幾歲的女兒在蘇格蘭，從以前的婚姻和一個4歲的女兒從他目前的婚姻。