The French weather service warned Wednesday of flash flooding risks across much of the south, where a historic drought has parched the rugged Mediterranean hills, a day after fierce rainstorms lashed much of the country.

Alert levels remained in effect for five southern departments hit by storms overnight that dumped two to four centimetres of rain in less than an hour, with some areas reporting up to seven or nine centimetres.

With the ground hardened by this summer's extreme drought -- July was the driest month recorded since 1961 -- much of the rain could not be absorbed by the soil and instead overflowed into streams and roads.

No injuries were reported, but public transport was disrupted including in Paris, where videos of inundated metro stations swamped social media.

"The storms are making their way east... and can be violent with intense rain that in some places could see up to eight centimetres fall in a short amount of time" and potentially causing tornado-like conditions, the Meteo France weather agency said.

Authorities also urged people to postpone travel plans if possible, and to avoid waterways or trying to seek shelter under trees.

"Just 30 centimetres of water is enough to sweep away a car," the prefecture of the Var department on the French Riviera said.

The rain was nonetheless a relief for firefighters who have been battling a string of wildfires across France, in particular in the southwest, with most blazes now contained after burning thousands of hectares.