Islamic State militants released a video on Saturday which purported to show the beheading of Japanese journalist Kenji Goto, who the al Qaeda offshoot had been holding hostage.
Reuters could not immediately verify the video, which showed a hooded man standing over Goto with a knife held to his throat, followed by footage of a body with a head placed on it.
Japanese broadcaster NHK said Japan's government was trying to authenticate the video, which made no mention of a Jordanian pilot which Islamic State has also been holding captive. The White House said it was also working to confirm the video's authenticity.
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Addressing Japanese Prime Minister Shinzo Abe, the militant in the video said: "Because of your reckless decision to take part in an unwinnable war, this knife will not only slaughter Kenji, but will also carry on and cause carnage wherever your people are found. So let the nightmare for Japan begin."
The militant had the same British accent as the man featured in previous Islamic State videos showing beheadings. Goto wore an orange jumpsuit like Islamic State captives in past footage.
The landscape in the video showed a hill and land covered in scrub, and appeared different to the desert setting of previous videos.