The Croatian prime minister is to address the nation following reports that militants affiliated to the Islamic State (IS) group have killed a Croatian national.
There has been no official confirmation of the death of Tomislav Salopek.
A photo purporting to show his body was posted on Twitter by a user associated with the jihadist group Sinai Province.
Last week, Sinai Province said it would kill Mr Salopek if Egypt did not release imprisoned "Muslim women".
The photo circulated online on Wednesday shows a decapitated body in what appears to be desert, beside a knife driven into the sand and the black banner used by IS.
The caption says the Croatian, aged about 30, was killed "for his country`s participation in the war against Islamic State".
Croatia`s foreign ministry said it could not confirm Mr Salopek`s death, but Prime Minister Zoran Milanovic will hold a news conference at 17:00 (15:00 GMT). President Kolinda Grabar-Kitarovic has also said she has cancelled all her pre-arranged activities on Wednesday.
Mr Salopek, a father of two, had been working as a surveyor in Egypt for the French geoscience company, CGG, which is involved in the oil and gas industry.
He was abducted while travelling in a car on the Oasis Road, about 22km (14 miles) west of Cairo, on 22 July, security sources told the AFP news agency.
The Egyptian authorities mounted a frantic search for him but were unable to establish for whether he was in the Sinai peninsula, Egypt`s Western Desert or even across the border in Libya.
Last Wednesday, Sinai Province posted a video showing a man who identified himself as Mr Salopek, wearing an orange-coloured jumpsuit and kneeling in the desert next to a masked militant holding a knife.
Speaking in English, the man said that Sinai Province wanted to "substitute" him with Muslim women in Egyptian prisons, and that the Egyptian government had 48 hours to comply before he was killed.
As many as 40,000 Muslim Brotherhood supporters and other Islamists are believed to have been detained and more than 1,000 others killed in a crackdown by the Egyptian authorities since the military overthrew President Mohammed Morsi in July 2013.
If Mr Salopek`s death is confirmed, it would be the first time that jihadist militants in Egypt have beheaded a Western hostage since stepping up their a campaign against the state two years ago.
Sinai Province has claimed it was behind the killing of the American petroleum engineer William Henderson, whose body was found in a car in the Western Desert in August 2014.
The jihadist group was known as Ansar Beit al-Maqdis until it pledged allegiance to IS in November and changed its name.
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