Egypt's IS-affiliated Sinai Province announced on Wednesday that it has killed Croatian hostage Tomislav Salopek.
Ansar Beit Al-Maqdis published a graphic photo showing the victim's decapitated body on IS-affiliated Twitter accounts Wednesday afternoon.
The militant organisation gave the Egyptian government a 48-hour deadline on Wednesday threatening to kill Salopek if "Muslim women prisoners" in Egyptian jails are not released.
The deadline expired on Friday.
The photo, which could not be immediately verified by Ahram Online, carried the following caption in Arabic:
"The Croatian captive has been killed because his country participated in the war against the Islamic State. The deadline has passed and the Egyptian apostate government as well as his own government abandoned him."
The photo also included screenshots from news websites displaying headlines like "Croatia supports Egypt's war on terrorism" and "Croatia asserts its continual support for Kurdistan.”
The ultimatum was released in a video clip where Salopek, wearing an orange jumpsuit, and kneeling at the feet of a hooded man who was holding a knife, read the IS group demands in English in an unknown desert location.
The 31-year-old married father of two was reportedly abducted in western Cairo in July. Salopek was working as a topographer in Egypt with French geoscience company CGG.
Sinai Province, formerly known as Ansar Beit al-Maqdis, pledged allegiance to the IS group in November of last year.
The group has killed hundreds of security personnel in Sinai and other areas. The group has also claimed responsibility for the deadliest attacks against police and army forces outside Sinai including the bombing of Cairo security directorate in January 2015 and the attempted assassination of the former minister of interior in Cairo in 2013.
Although foreigners have largely been spared from the violence, Salopek was the second foreigner to be killed by the militant group, as the group also claimed responsibility for the abduction and killing of US oil worker William Henderson in late December.
Egypt designated the Sinai based Ansar Beit Al-Madqis as a terrorist organization in 2014.
The Croatian foreign minister had visited Cairo on Thursday in an attempt to find a way to end Salopek's ordeal.
The Egyptian government had publicly said it is doing its best to secure Salopek's release.