An image grab allegedly shows members of the Islamic State (IS) jihadist group, with among them a jihadist believed to be French citizen Maxime Hauchard (R). (AFP)
A young Frenchman was one of the jihadists in an Islamic State video that appeared to show the beheading of Syrian soldiers and a US aid worker, prosecutors said on Monday.
The man is believed to be 22-year-old Maxime Hauchard from a small village in Normandy in northern France.
He is thought to have "left for Syria in August 2013 after a stay in Mauritania in 2012," Interior Minister Bernard Cazeneuve told reporters.
The Paris prosecutor's office said "circumstantial evidence confirms the involvement of a Frenchman in the decapitation of Syrian prisoners shown in an Islamic State video released on Sunday."
An investigation into Hauchard was opened in August by French authorities "for criminal association in relation to a terrorist organisation," a judicial source said.
The video released on Sunday shows the young, bearded man in camouflage fatigues alongside fellow combatants.
Prisoners are lined up on their knees in front of them. The man resembling Hauchard is seen placing a knife to the collar of one of the prisoners. The execution itself is not shown, but the severed head is later seen.
Hauchard was interviewed in July by French TV station BFM. He spoke over Skype from the Syrian city of Raqqa -- an IS stronghold -- saying he had travelled from Paris via Istanbul.
He said the ticket was "not expensive" and he had taken no precautions to hide his journey.
Converted to Islam at 17, he told the station he had decided to join the jihad after watching videos online.
He said he travelled alone and had quickly been taken under the wing of IS once he entered Syria.
"To show allegiance, you must first go to a training camp. The first stage lasts around a month. We do some training, we go on operations and after we return to training. It's not just theory," he told BFM.
He said he lived in a barracks with around 40 other jihadists -- mostly Arabs from Egypt, Morocco and Algeria, but also French.
"The personal objective of everyone here is shahid (martyrdom). That is the greatest reward," he said.
An intelligence source said it was being verified whether a second French citizen was among the jihadists seen in the video.
Around a thousand French are thought to have taken part in the conflict in Syria and Iraq, with around 375 currently there, the government has said. At least 36 have died there.
Overall, some 3,000 Europeans had joined the jihadists, EU counterterrorism chief Gilles de Kerchove said in September.