Islamic militants released a video showing the beheading of a man they said was kidnapped US journalist James Foley, and threatened to kill a second American unless President Barack Obama calls off air strikes on Iraq.
After the video was released on Tuesday, Foley's family confirmed his death in a statement posted on a Facebook page that was created to rally support for his release, saying they "have never been prouder of him".
British Foreign Secretary Philip Hammond voiced his "absolute horror" at the beheading and said the executioner in the video appeared to be British, and that work was under way to verify his background.
Foley, a 40-year-old journalist from Rochester, New Hampshire, went missing in northern Syria while freelancing for Agence France-Presse and the Boston-based GlobalPost.
The car he was riding in was stopped by four militants in a zone that both Sunni rebel fighters and government forces had been trying to control.
The video released on websites on Tuesday shows the sophistication of the Islamic State group's media arm and begins with scenes of Obama explaining his decision to order air strikes.
It then cuts to a bald man in a jumpsuit kneeling in the desert, next to a black-clad militant with a knife to his throat. Foley's name appears in both English and Arabic graphics on screen, and he is wearing a clip-on microphone as he begins his statement. The scene is captured on at least two video cameras and has been edited in a professional style.
After the captive speaks, the masked man is shown apparently beginning to cut at the neck of the captive. The video fades to black before the beheading is completed.
The next shot appears to show the captive lying dead on the ground, his head on his body. The video appears to have been shot in an arid area and the sound quality is sharp.
At the end of the video, a militant shows a second man, who was identified as another American journalist, Steven Sotloff, and warns that he could be the next captive killed. Sotloff was kidnapped near the Syrian-Turkish border in August last year and freelanced for Time and other media.
Obama was briefed about the video as he flew from Washington to resume his vacation on the resort island of Martha's Vineyard in Massachusetts.
Officials from the State Department and Pentagon contacted social media sites on Tuesday to inform them of the video and ask them to remove it. National Security Council spokeswoman Caitlin Hayden said the administration asked the sites to "take appropriate action consistent with their stated usage policies".
The beheading marks the first time Islamic State has killed an American citizen since the Syrian conflict broke out in March 2011.
On a website maintained by supporters of the Foley family, freejamesfoley.org  a message identified as from the journalist's mother, Diane Foley, spoke of his death.
"We have never been prouder of our son Jim," the message said. "He gave his life trying to expose the world to the suffering of the Syrian people. We implore the kidnappers to spare the lives of the remaining hostages."
Watch: Parents of slain journalist Foley praise their 'fearless' son
In 2011, Foley was among a small group of journalists held captive for six weeks by the government in Libya.
In a May 2011 interview about his experience, he recounted watching a fellow journalist being killed in a firefight and said he would regret that day for the rest of his life.
At the time, Foley said he "would love to go back" to Libya to report on the conflict and spoke of his enduring commitment to his profession.
"Journalism is journalism," Foley said during an interview, which was held in GlobalPost's office in Boston.
"If I had a choice to do Nashua [New Hampshire] zoning meetings or give up journalism, I'll do it. I love writing and reporting."
News of Foley's apparent execution comes as US air strikes appeared to yield some results, helping Kurdish and federal forces push Islamic State fighters back from some recently-conquered areas in northern Iraq, including the Mosul dam.
Watch: Expert: Radical groups like ISIS weaken the Muslim world
Associated Press, Reuters, Bloomberg, Agence France-Presse