Carolyn Kaster, AP
President Barack Obama salutes as he steps from Marine One onto the South Lawn of the White House, in Washington, Sunday, Nov. 16, 2014, as he arrives from Brisbane, Australia, by way of Hawaii and Andrews Air Force Base, Md.

GAZIANTEP, Turkey — Islamic State militants released a chilling videotape Sunday showing they had beheaded a fifth Western hostage, an American aid worker the group had threatened to kill in retaliation for airstrikes carried out by the United States in Iraq and Syria.

President Barack Obama on Sunday confirmed the death of the aid worker, Peter Kassig, a former Army Ranger who disappeared more than a year ago at a checkpoint in northeastern Syria while delivering medical supplies.

Kassig “was taken from us in an act of pure evil by a terrorist group,” Obama said in a statement from aboard Air Force One that was read to the news media in Washington.

In recent days, U.S. intelligence agencies received strong indications that the Islamic State had killed Kassig, the group’s third American victim. The president’s announcement was the first official confirmation of his death.

“Today we offer our prayers and condolences to the parents and family of Abdul-Rahman Kassig, also known to us as Peter,” Obama’s statement said. The president used the Muslim name that Kassig adopted after his capture, making the point that the Islamic State had killed a fellow Muslim. He acknowledged the “anguish at this painful time” felt by Kassig’s family.

The footage in the video released Sunday was of poorer quality than some of the group’s previous, slickly produced execution videos.

The video shows a black-robed executioner standing over the severed head of Kassig. Though the end result of the footage was grimly familiar, it was strikingly different from the executions of four other Western hostages, whose recorded deaths were carefully choreographed.

In the clip released early Sunday, the Islamic State displays the head of Kassig, 26, at the feet of a man with a British accent, who appeared in the previous beheading videos and has been nicknamed Jihadi John by the British news media. Unlike the earlier videos, which were staged with multiple cameras from different vantage points, and which show the hostages kneeling, then uttering their last words, the footage of Kassig’s death is curtailed — showing only the final scene.

Analysts said that the change in the videos suggested that something may have gone wrong as the militants, who have been under sustained attack from a U.S.-led military coalition and have faced a series of setbacks in recent weeks, carried out the killing.