In a video posted by the IS affiliate in Egypt the kidnapped man was seen kneeling in a yellow jumpsuit by the feet of a masked man holding a knife.

Reading a note in English, the man said his name was Tomislav Salopek, a 30-year-old married father of two. He said he would be executed within 48 hours if the Egyptian government did not release Muslim women.

"They want to substitute me with Muslim women arrested in Egyptian prisons," the man read.

"If Egyptian authorities do not act, he said, "the soldiers from Wilyat Sinai will kill me." Wilyat Sinai is a name for the militants' Egyptian offshoot.

Croatia's foreign ministry confirmed last month that one of its nationals, with the same initials as Salopek, was kidnapped on July 22 while on his way to work. Militants were said to have stopped the man's car at gunpoint, forcing the chauffeur out before driving away with their hostage.

Salopek's employer Ardiseis Egypt, a unit of the French seismic survey group CGG, confirmed that the man in the video was their employee.

Thousands detained by Egypt

The reference to "Muslim women" is believed to refer to female Islamists who were arrested in a government crackdown on dissent. Thousands of Islamists and suspected supporters of the now-outlawed Muslim Brotherhood are being held by Egyptian authorities.

The Croatian Ministry of Foreign and European Affairs said it was working to resolve the situation, but unable to provide detailed information that might aggravate the situation. There was no immediate comment from Egyptian authorities.

Egypt has seen increasing violence since a military coup deposed Egypt's democratically-elected President Mohammed Morsi.

There have been subsequent attacks by Islamists extremists in the Sinai Peninsula and further afield, mainly aimed at security forces. However, foreign interests have also been increasingly targeted, including the Italian Consulate, which was rocked by a car bomb last month.

Days earlier, a bomb killed Egypt's Prosecutor General Hisham Barakat in an upscale Cairo neighborhood.

rc/jil (AP, AFP, dpa)