The Islamic State in Iraq and Syria posted a video on Tuesday that it said showed the beheading of James Foley, an American journalist who was kidnapped in Syria nearly two years ago, according to a transcript released by the SITE Intelligence Group.

The authenticity of the video, which was also posted on YouTube, could not be verified, and a telephone call placed to Mr. Foley’s family was not immediately returned. YouTube later took down the four-minute, 40-second video.

Titled “A Message to America,” the video shows the journalist kneeling in a desert landscape, clad in an orange jumpsuit — an apparent reference to the uniforms worn by prisoners at the American military detention camp in Guantánamo Bay, Cuba. Standing to his left is a masked ISIS fighter, who begins speaking in English, with what sounds like an East London accent. Pulling out a knife, he says that Mr. Foley’s execution is in retaliation for the recent American airstrikes ordered by President Obama against the extremist group in Iraq.

“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,” Mr. Foley says in the video, which was uploaded to the online account of the al-Furqan Media Foundation, according to SITE, an organization that follows jihadist groups. He ends saying that when American soldiers began dropping bombs on Iraq this month, “they signed my death certificate.”

On Tuesday night, Mr. Foley’s mother, Diane Foley, issued a statement on the Facebook page the family had created to publicize their son’s disappearance: “We have never been prouder of our son Jim. 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.”

Two weeks ago, in the wake of American-led airstrikes against the terrorist group, which was fanning out across Iraq, jihadists had taken to social media to call for attacks on American interests. In the three hours after the graphic video of Mr. Foley’s beheading was uploaded on YouTube, jihadists using the hashtag “#NewMessageFromISIStoUS” surpassed 2,000 tweets, according to a survey by SITE, with many fighters gloating over his death, and calling it just retribution for the air raids.

Image An image from a video posted by the Islamic State in Iraq and Syria, which the group said was the execution of James Foley.

Mr. Foley, 40, a freelance journalist who was working for GlobalPost, an online publication based in Boston, as well as for Agence France-Presse, disappeared in Syria on Nov. 22, 2012. He was held alongside several other Americans, whose families have asked for a news blackout.

The video concludes with the fighter threatening to kill Steven Sotloff, another American freelance journalist, who was being held alongside Mr. Foley. Mr. Sotloff is seen kneeling in the same position, in the same landscape and wearing the same style of orange-colored jumpsuit. “The life of this American citizen, Obama, depends on your next decision,” the fighter says.

Mr. Obama was briefed about the video by Benjamin J. Rhodes, the deputy national security adviser, on Air Force One as he returned to Martha’s Vineyard, according to Eric Schultz, the deputy White House press secretary.

In Washington, a National Security Council spokeswoman, Caitlin Hayden, said in a statement: “We have seen a video that purports to be the murder of U.S. citizen James Foley by ISIL. The intelligence community is working as quickly as possible to determine its authenticity. If genuine, we are appalled by the brutal murder of an innocent American journalist,” she said, using an alternative name for ISIS.

Reached by telephone, Philip Balboni, the chief executive and a founder of GlobalPost, said that the newsroom and Mr. Foley’s family were also trying to establish the veracity of the footage. “We are still evaluating the video at this time,” he said.

Mr. Foley, who was last seen in Binesh, Syria, was also abducted in Libya in 2011, where he was held for several weeks after running into troops loyal to Col. Muammar el-Qaddafi’s crumbling government.

He was among dozens of journalists — many of them freelancers without the formal backing of a news organization — who disappeared in 2012 and 2013 in Syria.