More than 6,000 hectares (15,000 acres) of tinder-dry forest have burned in just 24 hours in the so-called Landiras blaze, the largest of several that scorched the region last month.
It had been brought under control -- but not fully extinguished -- after burning nearly 14,000 hectares, before flaring up on Tuesday, forcing the evacuation of some 6,000 people.
The government said Interior Minister Gerald Darmanin would travel to the frontline of fire-fighting efforts in the southwestern French region of Gironde on Thursday.
No one has been injured but 16 homes were destroyed or damaged near the village of Belin-Beliet, and officials said six fire engines had been burned.
"The risks are very high" that parched conditions will allow the fire to spread further, said Martin Guespereau, prefect of the Gironde department.
"The weather is very unfavourable because of the heat, the dry air, the record drought and the fact that there is a lot of peat in the ground... the fire didn't go out in July, it went underground," he told journalists.
Darmanin said more than 1,000 firefighters were now battling the blaze, adding that investigators suspected arson may be involved.
"There were eight fires that erupted between 8:00 and 9:00 am (0600 and 0700 GMT) that erupted at intervals of a few hundred metres, which is extremely unusual," he said in Mostuejouls, north of the Mediterranean city of Montpellier, where another fire was raging in the Grands Causses natural park.
He also told reporters that Sweden and Italy would send fire-fighting aircraft to France within 24 hours to help.
"It's a major fire... much more intense and fast-moving" than at the height of the Landiras blaze last month, Marc Vermeulen of the regional fire-fighting authority told journalists.
"I opened the door last night and there was (a) red wall in front of us, the sky was roaring like the ocean," said Eliane, a 43-year-old at a temporary shelter for evacuees in Belin-Beliet.
For Christian Fostitchenko, 61, and his partner Monique, waiting at a martial arts dojo in nearby Salles, it was their second evacuation of the summer from their home in Saint-Magne.
"This time we were really scared -- the flames were less than 100 metres (328 feet) from the house," he said.
The fire was spreading toward the A63 motorway, a major artery linking Bordeaux to Spain, with thick smoke forcing the road's closure between Bordeaux and Bayonne.
France has been buffeted this summer by a record drought that has forced water-use restrictions nationwide, as well as a series of heatwaves that many experts warn are being driven by climate change.
Wildfires have also ignited in the dry hills of the southeast and even in the normally lush areas of Brittany along the coast.
On Wednesday, officials in western France said a wildfire near Angers and Le Mans has burned 1,200 hectares since Monday as nearly 400 firefighters struggle to contain it.
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