Up to 200 French militants have traveled to Syria in the past year to join a two-year-old rebellion against Assad, Le Monde reported on Saturday, citing figures from France's internal DCRI and external DGSE security services.
That number, which is higher than a previous estimate of about 50, includes fighters who took up arms with rebel groups like the al Qaeda-linked Nusra Front. About two dozen have already returned to France, Le Monde said.
The newspaper also quoted security sources as saying France was worried about the risk of attacks by those returning home from the conflict.
But security sources said they lacked the legal means to monitor returning fighters effectively as too little was known about the Islamist brigades they had joined to justify interrogation and close surveillance.
Many Western nations are braced for attacks by radicalized individuals carrying out spontaneous attacks like the killing on Wednesday of a British soldier by two men on a London street in the middle of the day.
France, which intervened in Mali in January to help the government drive back an offensive by Islamist fighters, has voiced concern about the risk of home-grown militants like Gilles Le Guen, arrested this month in Timbuktu, returning home to carry out attacks.
Al Qaeda's North African wing, AQIM, claimed coordinated attacks on a military base and French uranium mine that killed 24 soldiers and one civilian this week.
But so far there had been no sign of concern about attacks by jihadists returning from Syria. Although French authorities are concerned depending on the outcome of the civil war in Syria.
Le Monde reported that while some 20 French "jihadists" were back in France, only one had been incarcerated - a 25-year-old Frenchman of Korean origin named Flavien Moreau who fought with Islamist rebel group Ahrar Al-Cham in Syria.