CAIRO (AFP) - The Islamic State in Iraq and Syria (ISIS) group said Wednesday it has beheaded a Croatian hostage, and released a picture of a body after what would be the first such killing of a Westerner in Egypt.

Croatian Prime Minister told reporters he could not "confirm with 100 per cent certainty" that Tomislav Salopek, who worked for French company CGG, had been murdered.

"But what we see does not look good," he said.

Salopek was abducted last month west of Cairo.

The militants had issued a 48-hour deadline that expired on Friday threatening to kill him if Muslim women prisoners were not freed from Egyptian jails.

The authenticity of the picture could not be immediately verified.

Salopek's abduction and purported killing were unprecedented in Egypt, which is battling an ISIS insurgency in the eastern Sinai Peninsula.

Although ISIS' Egyptian affiliate has killed hundreds of policemen and soldiers since the army overthrew Islamist president Mohamed Morsi in 2013, the country had been spared the gruesome kidnappings and executions of foreigners by the militants in Iraq and Syria.

The group has in the past beheaded Bedouin in Sinai it accused of collaborating with the army.

State-run Croatian news agency HINA quoted a foreign ministry source Wednesday as saying it "does not have confirmation that abducted Croatian citizen Tomislav Salopek has been killed".

The picture was posted on ISIS-affiliated Twitter accounts with the caption: "Execution of prisoner from Croatia - which has participated in war on Islamic State - after deadline ended."

'A SATANIC ACTION'

The 31-year-old father of two was working with French geoscience company CGG when seized from a car some 22km west of Cairo, security sources told AFP.

His abduction has rattled foreigners working for multinationals and underscored the militants' reach, despite the massive military campaign against ISIS.

Egypt had said it was intensifying efforts to locate Salopek after ISIS released a video of him last Wednesday.

Cairo-based Al-Azhar, a prestigious Sunni Islam learning centre, condemned the reported beheading.

"The killing of the Croatian hostage is a Satanic action that has nothing to do with religions and traditions," Al-Azhar said in a statement.

In last week's video, Salopek, kneeling next to a masked militant holding a knife, was forced to read a statement saying he would be executed in 48 hours if Cairo failed to release female prisoners, a key demand of Islamist militants over the past two years.

Salopek's abduction had been treated by police as a criminal kidnapping before the video emerged.

The ISIS demand to free female prisoners appeared aimed at scoring propaganda points at the expense of the Islamist opposition in Egypt, which does not share the militants' tactics, an analyst said.

Militants and their supporters have mocked the more mainstream Islamist opposition, which denies that it uses violence and has condemned militant attacks.

"This is a calculated propaganda move by them," said Mokhtar Awad of the US-based think-tank Centre for American Progress.

"The jihadists' message is: 'We are the ones who can deliver results. We are the ones who are serious about this. More importantly, we are putting it front and centre in our campaign'," he said.

TARGETING ECONOMIC INTERESTS

The abduction also appears aimed at striking at Egyptian economic interests, said another analyst.

"For Egyptian authorities, this execution means a further radicalisation (of the Islamic State affiliate) and extending its actions to foreigners, implying a concentration on economic targets to further weaken the regime," said Mathieu Guidere, an expert with the University of Toulouse in France.

Salopek's father had appealled to the kidnappers to release him, and Croatian Foreign Minister Vesna Pusic visited Cairo for emergency talks.

In his home town, neighbours described Salopek as a friendly young man who had gone to Egypt to earn a living.

Egypt, led by President Abdel Fattah al-Sisi, had been at pains to persuade international investors and companies that the country was safe after two years of violence and militant attacks.

Sisi was the former army chief who overthrew Morsi, setting up a police crackdown on the Islamist's followers that killed more than 1,000 protesters.

Hundreds of people, including Morsi, have been sentenced to death, although most have won new trials.

Seven people have been executed.

In Sinai, which borders Israel and the Palestinian Gaza Strip, militants have waged a brazen insurgency that appears unabated despite a sweeping army campaign.

But ISIS has managed to stage attacks outside Sinai only a few times.

It claimed responsibility for a bombing that killed a passer-by last month at the Italian consulate in Cairo.

And last December, the group said it murdered an American who worked for petroleum company Apache, also west of the capital.