Leaders of Australia and the U.K., as well as a representative of the U.S. National Security Council, have publicly denounced the contents of a video that allegedly shows members of the Islamic State militant group in Syria beheading Abdul-Rahman Kassig, an American aid worker who was born Peter Kassig. The militant group formerly known as either ISIL or ISIS purportedly posted the video online Sunday; it appears to show so-called Jihadi John, a militant responsible for previous hostage beheadings, holding Kassig’s head.

Australian Prime Minister Tony Abbott denounced the footage at a press conference covered by the Guardian. Abbott would not comment on the video’s authenticity or whether he had seen any additional footage, saying only that the Islamic State group should be called a death cult.

“We all know that the ISIL death cult is utterly barbaric, utterly barbaric, and beheadings are part of their stock-in-trade. ... We have seen a gruesome catalog of beheadings, of crucifixions, of mass executions, of sexual slavery since the ISIL death cult started to consolidate its hold in eastern Syria and northern Iraq,” Abbott said. “I don’t have confirmation of authenticity of any new video, so I don’t want to comment on specifics, just to say that this is a death cult, it can’t be dignified with any other title.”

British Prime Minister David Cameron posted on Twitter earlier Sunday, saying  he was horrified by what he called Kassig’s “cold-blooded murder”:

Meanwhile, U.S. National Security Council representative Bernadette Meehan issued a statement saying the council was looking to authenticate the video.

“We are aware of a video that claims to show the murder of U.S. citizen Peter Kassig. ... The intelligence community is working as quickly as possible to determine its authenticity,” NBC News quoted Meehan as saying. “If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends.”