French authorities have foiled a plot by four would-be extremists to attack military bases, the interior minister said Wednesday.

The suspects, aged between 16 and 23, were arrested Monday at four separate locations around France, Interior Minister Bernard Cazeneuve said.

Cazeneuve said the group's leader had been under surveillance after his online activities raised investigators' suspicions. He had also triggered alarms by signaling his desire to travel to Syria, Cazeneuve said. One of the suspects was a former member of the French navy, Cazeneuve said.

The minister said authorities break up similar plots every week in France, where more than 300 people have been implicated in jihadi activities since 2012.

There are no known links between the suspects arrested Monday and a double blast Tuesday at a petrochemical plant in southern France, Cazeneuve said.

A total of 1,850 French citizens or permanent residents are implicated in jihadi networks, and 500 are believed to be currently in Syria or Iraq, Cazeneuve said.