The two girls, aged 16 and 17, were stopped at Marseille airport as they prepared to board a flight to Istanbul.
Police say that say that several clues led them to conclude their eventual destination was Syria;
The two, who lived in different parts of France and had become friends via the internet, were returned to their families.
Also on Saturday five people detained during the week in eastern France were charged with plotting terrorist activities and financing terrorism and kept in detention.
Two of the suspects, who are brothers, “nurtured the ambition of dying as martyrs”, an official statement said.
Three of their brothers are currently fighting in Syria, where a relation died in a suicide-bombing in June, while another three brothers live near the central city of Lyon.
Another two suspects, a brother and sister aged 22 and 24, were arrested on Tuesday but a third brother, aged 19, fled, only to be caught in a shop on the outskirts of Lyon with a Smith & Wesson pistol in his possession.
Police found a Kalashnikov assault rifle and munitions at his home, they say.
The group are accused of running a network to send would-be jihadis, especially young women, to Syria and of planning an attack in France, although they are said not to have decided on a target.
A new anti-terror bill won overwhelmingly backed by the lower house of the French parliament on Thursday.