A Frenchman accused of helping a 14-year-old runaway girl try to join the Islamic State of Iraq and the Levant (ISIL) by arranging her travel is on trial on terrorism charges.

Friday's trial of Riad Ben Cheikh sheds light on the shadowy networks that recruit European adolescent girls to join fighters in Syria.

The girl made Facebook contact with an ISIL recruiter who persuaded her to join him in Syria, where he promised to marry her and teach her to fight.

Ben Cheikh reportedly booked a hotel and paid her fare to the airport, where she was detained in line to board a flight to Istanbul.

France has stepped up its crackdown on supporters of armed groups like ISIL and al-Qaeda since a number of attacks, including the deadly shooting at the offices of the satirical newspaper Charlie Hebdo, rocked the country in January.

European countries have been struggling to deal with a wave of residents who have either travelled to the Middle East to join armed groups or attempted to.