A second Frenchman was identified Wednesday after appearing unmasked in a grisly execution video among Islamic State jihadists, many of whom are believed to be Western fighters.
Several European countries are probing the possible involvement of their nationals in the execution of 18 Syrian prisoners and kidnapped US aid worker Peter Kassig.
The video sparked global horror after its release Sunday by the IS Sunni Muslim extremist group that has carried out widespread atrocities since seizing control of large parts of Iraq and Syria.
One foreigner, 22-year-old Maxime Hauchard from Normandy in northern France, was quickly identified by French prosecutors.
He is one of several young French nationals from a middle class, atheist or Catholic background to have converted to radical Islam and gone to fight in Syria, making a profile of potential jihadists nearly impossible to pin down, experts say.
A source close to the French investigation on Wednesday identified a second national as 22-year-old Mickael Dos Santos from an eastern Paris suburb, who goes by the name of Abu Othman.
He is believed to have left for Syria in the autumn of 2013, and became known to investigators shortly afterwards during a probe into a network channelling jihadists into the war-torn country.
"The man concerned is known for his terrorist involvement in Syria and his violent behaviour shown on social networks," Prime Minister Manuel Valls said, without confirming his identity.
In July, Hauchard said in an interview with French television he had decided to join IS after watching videos online.
"The personal objective of everyone here is (to become a) shahid (martyr).
That is the greatest reward," he said.
Thousands of foreign fighters have flocked to join IS in Iraq and Syria, and experts say they are often among the most violent and brutal of the jihadists.
A British-accented militant nicknamed "Jihadi John" has been at the centre of previous IS beheading videos and appeared again in Sunday's recording claiming Kassig's killing.
The video also showed the Syrian men kneeling on the ground each before a separate executioner, whose faces were uncovered.
Other known foreign fighters are believed to have appeared in the video, including an Australian and a Dane.
A Belgian newspaper reported that one of the men featured in the video looked like Abdelmajid Gharmaoui, who is currently on trial in his absence in Belgium for membership of a jihadist group.
Around 1,000 French nationals are thought to have taken part in the conflict in Syria and Iraq, with 375 currently there, the government has said.
At least 36 have died.
French President Francois Hollande, on a visit to Australia, said the issue of foreign fighters and how they were being "brainwashed" was a major concern.
"They could be from any background, from any ethnic origin -- but they can easily be brainwashed into becoming converts, and this is a very important matter," he said.
"We must be vigilant, and we must be strong."
Earlier this month, Paris adopted an anti-terrorism law which will slap a travel ban on anyone suspected of planning to wage jihad, and Hollande said it was crucial that the full force of the law is used on those opting to fight overseas.
"What is very important is not just reducing the number and avoiding new ones (foreign fighters), but have some sort of penalty applying to those who went to these areas because if you go to a combat zone it is something that should be subject to sanctions and penalties within the law," he said.
Kassig, who took the name Abdul-Rahman after converting to Islam, was captured last year and became the fifth Western hostage beheaded by IS after two US reporters and two British aid workers.
Monitoring group the Syrian Observatory for Human Rights said Monday it had documented the execution of 1,429 people in Syria by IS in the five months since it declared the establishment of a "caliphate" in areas under its control. (AFP)
