"Circumstantial evidence confirms the involvement of a Frenchman in the decapitation of Syrian prisoners shown in an Islamic State video released on Sunday," the prosecutor's office in Paris said. 

In the graphic 16-minute-long video that was posted online on Sunday, the Islamist group shows the beheading of at least 18 Syrian soldiers as well as the severed head of 26-year-old US aid worker Peter Kassig who was abducted in Syria on October 1st, 2013. US President Barack Obama confirmed Kassig's death later on Sunday, calling it "an act of pure evil".

Earlier Monday, French Interior Minister Bernard Cazeneuve had said there was "a very strong probability that a French citizen was directly involved in these despicable crimes".

The minister identified the man as 22-year-old Maxime Hauchar from Normandy in northwestern France, adding that the suspect "left for Syria in August 2013 after a short stay in Mauritania in 2012".

An intelligence source told AFP that agents are now trying to verify whether a second French national also took part in the beheadings.

Hauchard, who according to French daily Le Figaro goes under the name of Abou Abdallah Al Faransi which means "the Frenchman" in Arabic, participated in a BFM TV news report in July in which he described how he had converted to Islam at the age of 17 after learning about the religion by watching videos online. In the news report he said he travelled to Syria on his own via Turkey, ending up in Raqqa, which is now the ISIS group's stronghold in Syria.

In the BFMTV interview, Hauchard said he was currently involved in a special mission in which he would be prepared to die as a martyr.