The drones were detected above the base on Ile Longue, a peninsula off the Brittany coast in northwestern France, at around 7:30 pm (1830 GMT) Thursday, said the source close to the operation.

An anti-drone and search operation was launched by the marine battalion, which protects the base, the source said. It was not immediately clear what those measures entailed.

A judicial investigation into Thursday's incident was to be opened by the military prosecutor's office in Rennes, the Atlantic Maritime Prefecture told AFP.

"Sensitive infrastructure was not threatened," Guillaume Le Rasle, spokesman for the maritime prefecture, told AFP.

He said it was "too early to determine" the origin of the drones, adding however that those flights were "intended to cause concern among the population".

Mysterious drone flights over airports and sensitive military and industrial sites have rattled nerves in Europe in recent months.

Concerns are growing that such disruptions could be part of Russian hybrid war tactics three-and-a-half years into its invasion of Ukraine, which is backed by the European Union.

Detecting the drones, making them non-operational by jamming them, or even shooting them down, are all complex and hazardous tasks. And while Russian involvement is suspected, it is difficult to prove.

France and the United Kingdom are the only European countries other than Russia with nuclear weapons.

France has maintained a sea-based nuclear deterrent force since 1971.

The Ile Longue base is home to France's four ballistic missile submarines, at least one of which is permanently at sea to ensure nuclear deterrence.

Drone flights in the restricted area are not uncommon.

"There are precedents," said Le Rasle.

On the night of November 17 to 18, a drone flight over the Crozon peninsula, which includes Ile Longue, was reported, but no military installations were flown over.