LONDON — Islamic State militants on Saturday released a video showing the execution of British aid worker David Haines in the same grisly manner as the killing of two American journalists in recent weeks, along with a threat to kill another Briton.
The video, titled “A Message to Allies of America,” shows Haines clad in an orange jump-suit kneeling beside a man who speaks in the same London-accented English as the apparent executioner in two previous videos.
The masked man addresses British Prime Minister David Cameron, telling him that the killing is retribution for “your evil alliance with America, which continues to strike the Muslims of Iraq.”
The video ends with a threat to kill a man, Alan Henning, who the Islamic State claimed is another British hostage of the terrorist group.
Cameron tweeted a statement late Saturday saying that “the murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude.” Cameron also vowed to “do everything in our power to hunt down these murderers and ensure they face justice, however long it takes.”
The killing of Haines, a 44-year-old father of two from Scotland who was working for the French aid agency ACTED, is likely to intensify calls in Britain for more direct action against the Islamic State.
Britain has previously said it is sending arms to Kurdish fighters battling the group and is supporting American airstrikes with surveillance and intelligence.
Britain has so far refrained from carrying out airstrikes itself. But Cameron and his allies have suggested that they might be open to direct British military intervention.
The video of Haines’s killing, which was first reported by the SITE Intelligence Group, was released just days after President Obama gave a prime-time speech from the White House in which he described an open-ended campaign to combat the Islamic State both in Iraq and in Syria.
The president said the United States would work in concert with “a broad coalition” of allies, including Britain.
Obama released a statement late Saturday condemning Haines’s killing: “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.”
The Islamic State has released two previous videos showing the execution of the American journalists James Foley and Steven Sotloff. The group threatened to kill Haines at the end of the Sotloff video, which was released less than two weeks ago.
On Friday, Haines’s family urged his captors to contact them.
“We have sent messages to you to which we have not received a reply,” the family statement said. “We are asking those holding David to make contact with us.”
Haines was abducted in March 2013 near the Atmeh refugee camp along the Turkish border in the Syrian province of Idlib. He was a veteran of the Royal Air Force, and since leaving the armed forces he had used his security expertise to work for aid agencies in conflict zones.
The British newspaper the Telegraph quoted his wife, Dragana Haines, as saying last week that their 4-year-old daughter had been asking about her father every day for the past year and a half.
“He’s everything to us. He’s our life. He’s a fantastic man and father,” she told the newspaper in an account published early Sunday. “Nobody can understand how we are feeling.”
Haines’s brother, Mike Haines, released a statement early Sunday saying David Haines “was and is loved by all his family and will be missed terribly.”
Mike Haines said his brother had been an aircraft engineer with the Royal Air Force, and after leaving, had been energized by a new career working for aid groups in conflict zones.
“David was most alive and enthusiastic in his humanitarian roles,” said the statement, which was released via the British Foreign Office. “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.”