The Islamic State militant group claimed Tuesday to have beheaded an American photojournalist in retaliation for U.S. airstrikes in Iraq.

A video posted on YouTube, later removed, purported to show the execution of James Foley after he recited a statement in which he called the U.S. government “my real killers.” A second prisoner, said to be Steven Joel Sotloff, like Foley an American journalist who disappeared while covering Syria’s civil war, then appears in the video.

The masked executioner, speaking in English with what sounds like a British accent, identifies Sotloff and says that “the life of this American citizen, Obama, depends on your next decision.”

American intelligence officials believe the video is authentic but are continuing to evaluate it, senior U.S. officials said Wednesday morning.

In a statement Tuesday, National Security Council spokeswoman Caitlin Hayden said of the video: “If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends. We will provide more information when it is available.”

The Islamic State released a video claiming it executed Foley.

President Obama was briefed on the video aboard Air Force One as he returned to his Martha’s Vineyard vacation and will be updated on further developments, the White House said.

British Prime Minister David Cameron cut short a family vacation and returned to London to chair emergency meetings on Iraq and Syria.

In an interview with the BBC, Foreign Secretary Philip Hammond acknowledged that the apparent executioner spoke with a British accent and said the video seemed to be genuine.

Hundreds of Britons are believed to have traveled to Syria to fight in the country’s civil war, including many who have joined the Islamic State.

“We’re absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities,” Hammond said. “Many of these people may seek at some point to return to the U.K., and they would then pose a direct threat to our domestic security.”

A European intelligence official said the British government was examining the video, and the speech of the purported executioner, to compare it with former Guantanamo Bay prisoners and other British residents believed to have joined the Islamic State.

Both prisoners in the video are wearing orange shirts and pants, similar to orange jumpsuits worn by detainees at the U.S. military prison at Guantanamo Bay, Cuba. A similar outfit, believed to be a jihadist symbol of the prison, was worn by Nicholas Berg, an American businessman kidnapped in Iraq in 2004 whose execution by an Islamic State precursor organization was recorded on video and posted online.

Foley, 40, was working in Syria for the Boston-based news Web site Global­Post when he disappeared on Thanksgiving in 2012.

Philip Balboni, GlobalPost’s chief executive and co-founder, said in a statement: “On behalf of John and Diane Foley, and also GlobalPost, we deeply appreciate all of the messages of sympathy and support that have poured in since the news of Jim’s possible execution first broke. We have been informed that the FBI is in the process of evaluating the video posted by the Islamic State to determine if it is authentic. Until we have that determination, we will not be in a position to make any further statement. We ask for your prayers for Jim and his family.”

In a statement Tuesday night on a Facebook page dedicated to his freedom, Foley’s mother appeared to accept that the video was authentic. “We have never been prouder of our son Jim,” Diane Foley wrote. “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. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world.”

Praising James Foley as “an extraordinary son, brother, journalist and person,” she asked that the family’s privacy be respected.

Sotloff, a freelancer who worked for several news organizations, disappeared in Syria in August 2013.

In addition to Foley and Sotloff, at least three other Americans are believed to be captives in Syria, including Austin Tice, a freelance journalist whose articles appeared in McClatchy publications and The Washington Post before his disappearance in August 2012, according to a 2013 GlobalPost article. No one has claimed to be holding them.

According to the Committee to Protect Journalists, at least 66 journalists, all but 10 of them Syrian, have been killed covering the Syrian war. If the video is authenticated, Foley would be the first American journalist known to be executed since the conflict began in early 2011. The video evoked the 2002 taped execution in Pakistan of Wall Street Journal correspondent Daniel Pearl by al-Qaeda.

The Islamic State, an offshoot of al-Qaeda, is the most powerful among a number of extremist organizations that have emerged during the Syrian civil war, which began as a popular uprising against President Bashar al-Assad. Fighting against both Assad and U.S.-backed rebels, the militants now control much of eastern Syria and claim to have established an Islamic caliphate spanning Syria and neighboring Iraq.

As the group has grown, it has merged with the group formerly known as al-Qaeda in Iraq, under the leadership of Abu Bakr al-Baghdadi. According to U.S. intelligence, it numbers in the thousands, including foreign fighters from Europe and the United States.

In April, Islamic State fighters swept across the border into northern Iraq, taking over the city of Mosul before moving southward to within 60 miles of Baghdad. Extensive reports of executions, including beheadings and crucifixions, have emerged from areas under the group’s control.

This month, amid reports of stranded and besieged Iraqi minorities threatened with execution, the militants advanced eastward toward Irbil, the capital of the semiautonomous Kurdish region of Iraq.

On Aug. 7, Obama authorized U.S. airstrikes to rescue stranded minorities and protect U.S. personnel and facilities in Irbil and Baghdad. On Monday, after a total of 68 strikes from jets, bombers and drones, Obama announced that Iraqi and Kurdish forces, with U.S. air support, had retaken a strategic dam north of Mosul from the militants and that they had been pushed back from Irbil.

Within hours of that announcement, the Islamic State posted an online message warning it would attack Americans “in any place” in response to the airstrikes. “We will drown all of you in blood,” it said.

The title of the video posted Tuesday was “A Message to America” and was produced by the Islamic State’s media arm, according to the Site Intelligence Group, which monitors extremist Web sites.

A masked man dressed in black is shown standing in an unidentified desert location beside a prisoner kneeling beside him with his hands behind his back. “Obama authorized military operations against the Islamic State effectively placing America upon a slippery slope towards a new war against Muslims,” the masked man says.

The video then shows a clip of Obama’s Aug. 7 announcement, followed by a statement from the prisoner.

“I call on my friends, family and loved ones to rise up against my real killers, the U.S. government, for what will happen to me is only a result of their complacent criminality,” he says. He asks his parents not to accept “any meager compensation from the same people who effectively hit the last nail in my coffin with the recent aerial campaign in Iraq.”

The prisoner also appeals to “my brother John, who is a member of the U.S. Air Force,” to “think about what you are doing.”

“I wish I had more time,” he says. “I wish I could have the hope for freedom to see my family once again, but that ship has sailed. I guess all in all, I wish I wasn’t an American.”

The masked man then identifies the prisoner as “James Wright Foley, an American citizen of your country.” He then reaches down with a large knife and begins the apparent beheading of the prisoner; the screen fades to black and the next image is of a body with a head placed upon its chest.

The masked man then appears with another prisoner, identified as Sotloff, in a similar kneeling position.

Foley reported from some of the most dangerous recent crises and was imprisoned for 44 days in Libya in 2011 by forces loyal to deposed leader Moammar Gaddafi. According to Global­Post, two eyewitnesses saw his interception by a group of armed men in a silver-colored van on a road near the town of Taftanaz in northern Syria on Nov. 22, 2012.

Since then, Global­Post “has mounted an extensive international investigation . . . to determine who kidnapped Foley and where he was being held,” Global­Post said in an article on its Web site late Tuesday.

Julie Tate, Greg Miller and Dan Lamothe in Washington, Griff Witte in London and Souad Mekhennet in Frankfurt, Germany, contributed to this report.