Fire broke out onboard a nuclear submarine docked at the Mediterranean port of Toulon on Friday and was still raging but under control by afternoon, local officials said.
Nobody was hurt and no nuclear materials or weapons were on board the vessel, the Perle, which is in dry dock for repairs, the prefecture said in a statement.
"The fire is under control. It will not spread, but it has not yet been put out," a spokeswoman for the Mediterranean prefecture told Reuters.
The prefecture also said that smoke coming from the Perle was not radioactive.
Dozens of firefighters were on the scene, helped by specialised teams and a firefighting ship brought in from Marseille, the prefecture said.
Local newspaper Var-Matin reported on its website that the fire broke out at 10:30 a.m..
A defence ministry source said Defence Minister Florence Parly would go to Toulon as soon as possible.
Toulon is France's biggest naval base and is home to most of its fleet, including the nuclear-powered aircraft carrier Charles de Gaulle and the nuclear submarines and other warships that escort it on its missions.
The 74-metre-long (240-foot) Perle was launched in 1990 and started active service in 1993. It can be at sea 220 days per year.