France's interior minister on Sunday said he was alarmed at the growing number of French jihadists, including minors, going to fight alongside extremist groups in Syria.
Manuel Valls said the trend of young fighters heading for Syria had "accelerated" in recent weeks, and that around a dozen French minors had already gone to Syria or wanted to go there.
"The phenomenon worries me to say the least," Valls told French media. "It represents to me the greatest danger that we face in the coming years."
French authorities estimate that around 250 French nationals or residents are currently fighting alongside rebels against President Bashar al-Assad's regime.
Twenty-one have died in the fighting, while around 100 are thought to be on their way to Syria and 76 have already returned.
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Valls' comments came after prosecutors in the southwestern city of Toulouse on Friday raised the case of two 15-year-olds who had left France for Turkey with the aim of joining militia fighters in Syria.
The father of one of the teenagers has told French media that the boys had left on January 6 to link up with Al-Qaeda fighters after "being brainwashed on the Internet".
"It's not in the mosques that these recruitments are organised, it's most often online," Valls said in an interview with Europe 1 radio, I-tele television and the newspaper Le Monde.
Other European countries have also expressed concern about nationals joining the battles in Syria, where a nearly three-year-old war is estimated to have killed more than 130,000 people.
In December, the interior ministers of France and Belgium said between 1,500 to 2,000 young Europeans were thought to have teamed up with Al-Qaeda-linked rebel groups in Syria.
On Wednesday, the European Commission called for urgent action to tackle the problem, warning that youngsters who joined radical groups abroad posed a threat on their return home from combat zones.
Valls echoed that concern on Sunday. "The danger for us and our own interests lies in their return," he said. "These individuals have shown their willingness to fight with jihadist organisations. Their return is particularly tricky."