ISIS' affiliate in Egypt posted an image online Wednesday claiming to show the decapitated body of a Croatian hostage it threatened to kill last week.
A man identifying himself as 30-year-old geoscientist Tomislav Salopek appeared in a previous video in which militants threatened to kill him unless Egypt freed Muslim women prisoners.
The group, which was known as Ansar Bayt al-Maqdis before it pledged allegiance to ISIS, claimed Wednesday it had beheaded Salopek and posted what it said was a photo of his body, according to security consulting firm and NBC News partner Flashpoint Intelligence.
Salopek had been working as a contractor for French firm CGG when he was kidnapped on July 22.
Croatian Prime Minister Zoran Milanovic later announced he would address the nation at 5 p.m. local time (11 a.m. ET), a government spokeswoman told NBC News.
A Croatian Foreign Ministry spokeswoman said they had "no confirmation" regarding the hostage it has only referred to by the initials T.S.
The foreign ministry said in a statement late last month that "T.S." — who was born in 1984 — was kidnapped while driving to work. A group of armed men forced him to get out of the car and drove him to an unknown location, the ministry said.
It said it was "doing everything in its power" and working with the Croatian Embassy in Cairo to obtain information about the kidnapped man. "The ministry is also keeping close contact with the victim’s family," it said.
Egypt's Al-Azhar institute, the top center of religious learning for Sunni Muslims, condemned the apparent killing of Salopek as a "demonic act" contrary to all religion. In a statement, it added that what the "terrorists" did was a betrayal of the Prophet Muhammad and all Muslims.