The Islamic State in Iraq and Syria released a video Saturday of the third beheading of a foreign hostage, a British aid worker. The execution was a clear message to Britain, a vital ally of the United States as it builds an international coalition to target the militant group, which has made stunning advances across Syria and northern Iraq in recent months.

The video shows the aid worker, David Cawthorne Haines, kneeling on a bare hill under the open sky, in a landscape that appears identical to where two American journalists were killed by the group in back-to-back-executions in the past month. In the moments before his death, the 44-year-old Mr. Haines is forced to read a script, in which he blames his country’s leaders for his killing.

“I would like to declare that I hold you, David Cameron, entirely responsible for my execution,” he said. “You entered voluntarily into a coalition with the United States against the Islamic State.” He added: “Unfortunately, it is we the British public that in the end will pay the price for our Parliament’s selfish decisions.”

The video ends with the black-clad executioner, who was speaking in English with what sounded like a British accent, identifying their next victim as Alan Henning, another British citizen. SITE Intelligence, which tracks jihadist groups, said the executioner appeared to be the same man who killed the American hostages.