The Paris prosecutor's office said on yesterday that the 17-year-old boy was detained in the western town of Rennes and being questioned by a judge in Paris under a "criminal terrorist association" investigation.
The office says the teen used social networks to communicate with Rachid Kassim, who is thought to be operating from Syria or Iraq.
Kassim, whose name has appeared in connection with at least four recent terror plots targeting France, was the public face of an IS video claiming responsibility for the July 14 truck rampage that left 86 people dead in Nice.