CAIRO — A purported Islamic State group video released Wednesday threatened to kill a Croatian hostage if Egyptian authorities do not release “Muslim women” held in prison within 48 hours, a day before the country plans to unveil a highly promoted new extension of the Suez Canal.

The video, circulated on social media by Islamic State sympathizers, shows a man wearing a yellow jumpsuit kneeling in the desert before a knife-wielding masked man in military fatigues. An Islamic State flag flutters next to him and the video identifies it as coming from the Islamic State affiliate in Egypt’s lawless Sinai Peninsula.

Reading calmly from a note in English, the man identifies himself as Tomislav Salopek, a married, 30-year-old father of two, adding that Islamic State fighters captured him on July 22. If Egyptian authorities do not act, he said, “the soldiers from Wilyat Sina will kill me.” Wilyat Sina is the Arabic phrase for the Egyptian group calling itself the Sinai Province of the Islamic State.

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#ISIS threatens to execute Croatian captive in #Egypt if their damnds are not met in 48 hours – @EgyptianStreets pic.twitter.com/sGXwgJs90M — Conflict News (@rConflictNews) August 5, 2015

It was not clear where the video was shot. Nor was it exactly clear who the militants wanted released. Egypt, a majority Muslim country, now holds thousands of Islamists and suspected supporters of the outlawed Muslim Brotherhood group in prison. That follows the 2013 military overthrow of Islamist President Mohammed Morsi.

Croatia’s Foreign Ministry late last month said that one of its nationals with the same initials had been kidnapped in Cairo while on the way to work. The company, which Salopek identified as France’s CGG Ardiseis, works in the oil and gas sector and has a branch office in Cairo’s leafy suburb of Maadi, where many expats and diplomats live.

Calls to CGG Ardiseis’ office in Cairo were not immediately answered. Croatian authorities could not be immediately reached.

The footage, entitled “A Message to the Egyptian Government,” was shot in the style of previous Islamic State propaganda videos in which they threaten and behead hostages.