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Fears mounted for the fate of a Croatian abducted in Egypt by Islamic State group militants who had threatened to execute him by the end of Friday.

The unprecedented abduction in the North African country has rattled foreigners who flock to Egypt to work in multinational companies, while underscoring the extremists' reach despite a massive military campaign against them.

I am asking the people who hold my son to let him return to his family, his motive to go to your homeland was exclusively to earn bread for his children

Although Egypt is fighting an Islamic State group insurgency in the sparsely populated Sinai Peninsula, it had been largely spared the horrific kidnappings conducted by the Islamic State group in Syria, Iraq and Libya.

The father of Salopek, who works for the French geoscience company CGG, called on the kidnappers to release the 31-year-old father of two, as Croatian Foreign Minister Vesna Pusic left for Cairo to follow the case.

"I am asking the people who hold my son to let him return to his family, because his motive to go to your homeland was exclusively to earn bread for his children. Nothing else," Zlatko Salopek told AFP at the family's home in the eastern Croatian town of Vrpolje.

Salopek had appeared in an Islamic State video released over the internet on Wednesday, kneeling next to a masked militant holding a knife.

Reading from a paper, he said that his captors would execute him in 48 hours if the Egyptian government did not release female prisoners, which has been a demand of Islamist militants over the past two years.

Thousands of people, most of them Islamists, have been jailed since the army overthrew Islamist president Mohamed Morsi in 2013 and unleashed a deadly crackdown on his followers.

Salopek was abducted last month on a road running from the west of Cairo.

His driver was left unharmed, and police say they have questioned him.

It was not clear where the militants were holding Salopek in the vast and mostly desert country.

While the extremists mostly operate in Sinai, in northeastern Egypt, they have also conducted attacks in the country's western desert over the past two years.

Formerly known as Ansar Beit al-Maqdis, the group changed its name when it pledged allegiance to IS in November.

Last December, the extremists claimed they killed an American working for petroleum company Apache, also west of Cairo.

That attack was at first treated as a deadly carjacking by police.

In July, IS said it was behind a car bomb attack targeting the Italian consulate in Cairo - the first such attack against a foreign mission in Egypt since extremists began their campaign following the crackdown on Islamists.

The video of Salopek was released the day before a lavish ceremony at the Suez Canal to celebrate an expansion of the waterway, with a host of world leaders in attendance.

Egyptian President Abdel Fattah al-Sisi, who as army chief toppled Morsi, pledged at the ceremony to "defeat" the militants.

The army says it has killed more than 1,000 militants over the past few years, but the insurgency in Sinai appears unabated.

The extremists have killed hundreds of soldiers and policemen, and even destroyed a navy vessel with a wire guided missile last month.