The move comes just days after four French men were sentenced to up to seven years in jail for having tried to wage war abroad, at a time of concern over the mounting number of nationals going to fight in Syria.

The group is suspected of having organized training in the Paris area, and some of them were in contact with members of the radical, Al-Qaeda-linked Al-Nusra Front which is fighting the troops of Syrian leader Bashar al-Assad.

The four, aged 21 to 28, were charged with criminal conspiracy linked to a terrorist enterprise; financing terrorism, and fraud.

Some of those charged are also suspected of being in contact with so-called "facilitators" -- French people in Syria who reportedly smooth the arrival of nationals and deploy them to combat zones.

According to the source close to the case, who wished to remain anonymous, investigators are particularly interested in the group's contacts with a man from the southeastern city of Lyon.

He is currently in Syria, where he is sometimes referred to as the "emir" of French people fighting in the war-ridden country.

According to Interior Minister Manuel Valls, as many as 700 French nationals could have joined the fighting in Syria.

Authorities fear they could pose a major security threat if they return home as battle-hardened militants.

NJF/NJF