The submarine was under renovation in the southern base of Toulon when the blaze broke out at about 10:30 am (0830 GMT) on Friday in a hard-to-access part of the lower bow section, the navy said.

It then took around 100 firefighters and 150 support crew to put out the blaze by around midnight (2200 GMT Friday), the navy said in a statement.

"There was a fire, but no, there was not a nuclear accident," Defence Minister Florence Parly said.

Parly added that during the fire "measures were taken in the rear area to protect the nuclear reactor compartment".

The navy said earlier there were no casualties and no risk of radiation because the nuclear fuel had been removed during the renovation of La Perle (The Pearl), one of France's six nuclear attack submarines.

The extent of the damage -- and what started the fire -- were not immediately clear but an official with the Naval Group repair centre called the incident "serious."

The regional authorities said that pollution and radioactivity tests carried out by independent experts had not found anything out of the ordinary.

But French nuclear watchdog association CRIIRAD said that a nearby measurement found that radioactivity levels had oscillated at low levels for a few hours in the days leading up to the fire.

The NGO said the "troubling" phenomenon "could have several explanations," but that it did "raise questions".

Parly responded by saying that "a natural radioactive element was emitted in infinitesimal quantities before the fire," adding that it showed that "we have an extremely sensitive and effective detection system".

Parly said a damage analysis would determine whether the vessel can be repaired, adding that it was lucky that almost all of the sub's equipment was not on board.

A judicial investigation and technical probe have been launched.

The submarine, which entered service in 1993, docked at Toulon in January for 18 months of renovation work that was to keep it operational until the end of the decade.

The vessel, which can dive to 300 metres (985 feet) with a crew of 70, is an attack submarine used for tracking ships, escorting aircraft carriers, carrying out coastal intelligence missions, and deploying special forces.

In addition to its six nuclear-powered attack submarines, France also operates four nuclear ballistic missile submarines.

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