Defence Minister Catherine Vautrin praised the military personnel at the submarine base on Ile Longue, a peninsula off the Brittany coast in northwestern France, for "intercepting" the drones.

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.

At least five drones were detected above the base at around 1830 GMT on Thursday, said the source close to the case.

An anti-drone and search operation was launched by the marine battalion, which protects the base, the source added.

The maritime prefecture declined to say whether the military had used firearms or anti-drone jamming guns.

The Ile Longue base is home to France's four ballistic missile submarines -- Le Triomphant, Le Temeraire, Le Vigilant and Le Terrible.

At least one is permanently at sea to ensure nuclear deterrence.

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.

- 'Intended to cause concern' -

"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".

Vautrin said any overflight of a military base was prohibited in France.

"I would like to commend our military personnel at the Ile Longue base for intercepting the aircraft," she told broadcaster TF1.

"A complaint has been filed, an investigation is underway, and it is this investigation that will determine what this overflight was all about."

Detecting the drones, making them non-operational by jamming them, or even shooting them down, are all complex and hazardous tasks.

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.

France's ballistic missile submarines are equipped with M51 strategic missiles with multiple nuclear warheads.

The Ile Longue base is a closely guarded site, employing 2,000 people including 1,500 civilians who maintain the four nuclear-powered ballistic missile submarines.

It is protected by 120 maritime police officers in coordination with marines.

Drone flights are prohibited over the Crozon peninsula, which includes Ile Longue, as well as over a large part of the Brest harbor, in order to protect the military infrastructure located there.

However, 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 was reported, but no military installations were flown over.

Last month, French police launched an investigation after an unidentified drone was spotted flying over a police station and a train carrying battle tanks in the city of Mulhouse near the German border.

On Friday, Irish police said they were investigating the sighting of several unidentified drones in the skies as Ukrainian President Volodymyr Zelensky's plane landed in Dublin this week.

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TF1