TOKYO — The Japanese government expressed outrage at an image released Saturday that purported to show the decapitated body of one of two Japanese hostages captured by Islamic State militants, but the chief government spokesman did not confirm the authenticity of the video.

The kidnappers had threatened to kill the men if a Friday deadline passed for a $200 million ransom from Japan.

According to SITE Intelligence, an organization that tracks jihadist propaganda, the image was shown in a video posted on Islamic State-linked Twitter accounts. Al Furqan, an ISIS-linked website that has posted videos of beheadings in the past, had not released any video or message confirming the killing by midday Saturday.

An audio that accompanied the video said the dead hostage was Haruna Yukawa, 42, who has been described as an adventurer. The other hostage, Kenji Goto, 47, a journalist, appeared to be alive in the video. The two men’s fate became a fixation in Japan and a major challenge for Prime MinisterShinzo Abe. If the reports of the death prove to be true, Mr. Yukawa would be the first Japanese to be killed by the Islamic State, which has established a self-proclaimed caliphate that is financed partly by extortion and kidnapping of foreigners.