A video released by the Islamic State in Iraq and Syria on Saturday appeared to show a masked militant executing British aid worker David Cawthorne Haines.
The UK foreign office said that it is "working urgently to verify" the video.
The footage was shot in what seemed to be the same location as two other videos ISIS released this summer, of the beheadings of American journalists James Foley and Steven Sotloff. Like the journalists, Haines is dressed in an orange robe, The Wire reports. The militant who kills Haines appears to be the same man who beheaded Foley and Sotloff.
At the end of the video, a fourth hostage is shown. He is identified as Alan Henning, a British citizen.
President Obama released a statement about the video Saturday evening. "The United States strongly condemns the barbaric murder of U.K. citizen David Haines by the terrorist group ISIL," the statement read. "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."
British Prime Minister David Cameron responded to the situation in a pair of tweets: "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," and, "We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
Haines was working with Acted, a French humanitarian agency, in the Idlib province of Syria when he was taken hostage in March 2013. ISIS had threatened to kill Haines in the video that showed Sotloff's execution earlier this month.
In the video released Saturday, Haines recites a condemnation of the British government from a script: "You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor Tony Blair did, following a trend among our British prime ministers who can't find the courage to say no to the Americans."
The "coalition" likely refers to a broader effort on the part of the Obama administration to involve U.S. allies, including the U.K., in the fight against ISIS. President Obama said in a Wednesday night speech that the U.S. "will be joined by a broad coalition of partners" in its strategy to combat ISIS.
"Already, allies are flying planes with us over Iraq; sending arms and assistance to Iraqi security forces and the Syrian opposition; sharing intelligence; and providing billions of dollars in humanitarian aid," the president said.
Although British Foreign Secretary Philip Hammond said on Wednesday that the U.K. "will not be taking part in any air strikes in Syria," Cameron later clarified that Hammond was only referring to military action against Syrian President Bashar al-Assad's regime, a course of action that was voted down in the British Parliament last year.
Hammond also said that the U.K. "supports entirely the U.S. approach of developing an international coalition," and that the U.K. had "ruled nothing out" in the way of participating in that coalition.
President Obama's Saturday statement reaffirmed the coalition's goals. "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."