An Islamic State militant in Syria instructed one of the suspects in a foiled attack on a French military base to "hit" France, Paris prosecutor Francois Molins said on Friday.
The youngest of three suspects in custody — aged only 17 — was given the order when it became clear he would not be able to leave the country to wage jihad in Syria because he was under surveillance, Molins said.
Four young men, including a formal naval signalman, were arrested in dawn raids on Monday, suspected of "planning to commit a terrorist act" at a French military base in the south of the country.
The youngest, 16, was quickly released, but the other three are thought to have been plotting to kidnap and behead a member of the military on film. The trio was planning to carry out the attack around the end of December or beginning of January 2016, Molins said.
According to a security source, all three had been planning to travel to jihadist-controlled areas of Syria but the 17-year-old's mother became concerned about his radicalisation and contacted the authorities.
He was interviewed by counter-terrorism officials and was aware he was under surveillance.
"They claim to be part of Daesh," said a source close to the investigation, using an alternative Arabic acronym to refer to the jihadist Islamic State group that controls swathes of Syria and Iraq.
The three men are expected to be interviewed by a judge on Friday ahead of possible charges.
France remains on high alert more than six months after jihadist attacks in January that claimed 17 lives and started with shootings at satirical magazine Charlie Hebdo.
"We are facing a terrorist threat that we have never seen before — an external threat and an internal threat," said Prime Minister Manuel Valls on Thursday.
Although the foiled assault was planned around the first anniversary of the Charlie Hebdo attacks, none of the three alleged plotters indicated that the date was chosen deliberately for this reason.