The suspects planned to capture and behead a high-ranking soldier from a military base, newspaper Le Monde reported.

The four people arrested were aged between 16 and 23.

“The main instigator was detected after being active on social media and due to apparent links with French jihadists now in prison”.

All three of those still under arrest had been planning to travel to jihadist-controlled areas of Syria, the security source said, but the 17-year-old’s mother contacted authorities and he was interviewed by counter-terrorism officials.

France has remained high alert for terrorist attacks since the attack on the offices of satirical magazine Charlie Hebdo and a Jewish supermarket in January earlier this year, when a trio of gunmen killed a total of 17 people.

The arrests, made on Monday, were announced by President François Hollande and confirmed by the interior minister Bernard Cazeneuve.

The suspected plotters are now in the custody of France’s intelligence services, the DGSI, and anti-terrorist prosecutors in Paris have opened a probe, Cazeneuve said.

“This week, we stopped terrorist attacks which could have taken place”, Mr. Hollande said on a visit to the coastal city of Marseille. One of the members of the group once served in the French navy, they added.

Cazeneuve said Wednesday that 1,850 French citizens or people who usually reside in France are implicated in jihadist networks, including close to 500 individuals now in Syria or Iraq.

Authorities are still investigating the simultaneous blasts early Tuesday that hit two tanks containing chemicals, setting off huge fires at the plant in Berre-l’Etang, northwest of Marseille, according to CNN affiliate BFMTV.

Tuesday, explosions happened at a pair of petroleum tanks in southern France that officials believe may be terror-related – and possibly linked to the theft of explosive devices from a nearby military camp last week.

Paris tightened security around sensitive sites such as factories, calling for “maximum vigilance”.

Last month, the nation was shocked by an attack in which a man on France’s terror watch list allegedly decapitated his boss before launching an attempted suicide attack at a U.S.-owned chemical factory near Lyon.