PARIS, Jan 31 (Reuters) - A French court put two teenagers caught trying to join rebel groups in Syria under formal investigation on Friday on suspicion of planning terrorist acts - an attempt to stop a stream of volunteers by prosecuting them pre-emptively.
The boys, aged 15 and 16, were arrested in Turkey and escorted back to France where police in the southern city of Toulouse questioned them for 48 hours, the maximum for minors, before transferring them to Paris.
After further questioning, an investigating magistrate in Paris placed them under formal investigation for conspiring to commit terrorist acts. They are the youngest suspects to face prosecution on such a charge in France.
The case highlights French authorities' growing concern about men as young as 15 leaving France to join Islamist rebel groups fighting in Syria, a trend that has accelerated around Europe over the past year.
On Friday, French media reported that police were trying to ascertain whether a missing 15-year-old girl in the southern city of Avignon had also left for Syria.
Lawyers for the young men said they disputed the accusations, arguing that travel to Syria was not illegal and that the boys were not guilty of any wrongdoing.
"I dispute the formal investigation... There is no wrongdoing," said lawyer Agnès Dufetel-Cordier, who represents the 15-year-old known by his first name, Yacine.
Thousands of young men from Muslim immigrant backgrounds have left Europe to join rebels in Syria, worrying police and authorities who fear possible attacks in European capitals when radicalised, battle-hardened fighters return home.
However, judicial authorities have struggled to address the threat or prosecute volunteers for a rebellion against Syrian President Bashar al-Assad that in most cases is supported by their governments.
Interior Minister Manuel Valls said this week he would ask Prime Minister Francois Hollande to approve measures designed to help families and local authorities detect candidates for the war in Syria, including an anonymous tip hot-line.
He said that judges could prosecute those who have left if they planned to join Islamist rebel factions such as the Nusra Front, designated a terrorist group by the United Nations. (Reporting By Nicholas Vinocur and Gerard Bon)