The ISIS militant who appears in the terror group's newest propaganda video could be the notorious British executioner known as Jihadi John.

The masked extremist in the new video stands over a Croatian hostage, who they have threatened to kill if Egypt does not release 'Muslim women' held in its prisons within 48 hours.

Jihadi John has not appeared in one of Islamic State's propaganda videos since he was outed as Mohammed Emwazi from west London in February.

The ISIS fighter in the video does not speak but does bear some similarities to the man once described as Islamic State's chief executioner.

He holds a sharp blade in his left hand, just as Emwazi did before he beheaded American journalist Steven Sotloff, his countryman Peter Kassig and British aid workers David Haines and Alan Henning.

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Hostage: The video, purportedly released by ISIS, issues a threat threat to kill 30-year-old Tomislav Salopek

Gruesome: The masked fighter in the new video holds a sharp blade in his left hand, just as Emwazi did before he beheaded British aid worker Alan Henning (pictured) - and many others

The video, which was shared on Wednesday by Islamic State sympathisers, shows a man wearing a yellow jumpsuit kneeling before a knife-wielding masked man in military fatigues.

Reading from a note, the captive identifies himself as Tomislav Salopek from Croatia and says ISIS fighters captured him on July 22.

He says: 'My name is Tomislav Salopek, I am 30 years old and I am coming from Croatia... I am married and I have two children.'

The Croatian citizen was kidnapped while heading to work in a company car which belongs to French firm Compagnie Generale de Geophysique, who he worked for as a topographer.

Mr Salopek, 31, continues: 'They want to substitute me with the Muslim womens arrested in Egyptian prisons.

'This matter have to be achieved before 48 hours from now, if not the soldiers of Wilyat Sina will kill me.'

Weapon: The video says he can be saved if the Egyptian government releases female Muslim prisoners

The father-of-two was married to a woman named Natasa, who is from the Croatian capital of Zagreb.

His family was with him in Cairo, but they have moved back to Croatia since he was captured. Salopek was due to return with them - to their home in the eastern Croatian town of Vrpolje - just two days after he was abducted.

Croatian media has speculated that the extremists who captured him may have mistaken him for a French citizen.

Wilyat Sina is the Arabic phrase for the Egyptian group calling itself the Sinai Province of the Islamic State.

It was not clear where the video, entitled A Message to the Egyptian Government, was shot. Nor was it exactly clear who the militants wanted released.

Egypt, a majority Muslim country, now holds thousands of Islamists and suspected supporters of the outlawed Muslim Brotherhood group in prison. That follows the 2013 military overthrow of Islamist President Mohammed Morsi.