It was the latest in a series of mysterious drone flights over airports and sensitive military and industrial sites across Europe.

Three-and-a-half years into Russia's invasion of Ukraine, there is growing concern that such disruption could be part of hybrid war tactics by Moscow against the European Union, which has backed Kyiv.

No drones were shot down and no pilots were identified during the flyover Thursday evening at the submarine base on Ile Longue, a peninsula off the Brittany coast in northwestern France, prosecutors said as they opened an investigation.

"No link with foreign interference" has been established at this stage, said prosecutor Frederic Teillet.

The investigation had to "confirm whether or not these were drones" and determine "the type and number of devices," he added.

One source close to the case told AFP that five drones had been detected above the base at around 1830 GMT Thursday.

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

"The marines fired a jammer, not a firearm," said Teillet.

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.

- 'Intended to cause concern' -

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

Speaking earlier in the day, he said that it was "too early to determine" the origin of the drones, adding however that the flights had been "intended to cause concern among the population".

Defence Minister Catherine Vautrin told French broadcaster TF1 that any overflight of a military base was prohibited in France.

Praising the response of the military personnel at the base, she added: "A complaint has been filed, an investigation is underway, and it is this investigation that will determine what this overflight was all about."

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

Drone flights are prohibited over the Crozon peninsula which includes Ile Longue in order to protect the military infrastructure located there.

However, drone flights in the restricted area are not unprecedented, said Le Rasle.

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