The wildfire has since Wednesday ravaged 42,000 hectares (103,000 acres) of forest -- an area larger than the US city of Detroit -- west of Bordeaux. 

Tourists and residents have had to flee at the height of the summer holidays, while around 240 homes have been destroyed.

"The fire is not contained, which means it's still burning," Interior Minister Laurent Nunez told reporters in Paris.

"We really need to remain extremely cautious."

Drizzle overnight offered a little respite to firefighters, but all hands were on deck on Monday to stamp out as much of the blaze as possible before a new spike in temperatures on Tuesday.

Tractors fitted with blades to clear land, pickups towing water tanks, and logging trucks sped along a road on Monday to join a convoy of fire engines southwest of Bordeaux.

Farmer and wood transporter Christopher Bonnefond, 25, said he and three colleagues had left their homes in another part of France before dawn to provide support.

"We want to help," he said in the village of Marcheprime. "I'm a hunter and I want nature to stay like it is."

At a water point near the forest fire, he stood on top of a giant plastic cistern usually used for manure, watching a pipe fill it with water.

"It's a huge time-saver" for the firefighters, he said.

- Burnt-out car husks -

Further north in the forest, the road was lined with smouldering pine trees and blackened trunks, as firefighting planes flew overhead.

A specially modified A400M military transport plane dumped tonnes of fire retardant over the fire, the army said.

Three burnt-out car husks and a destroyed trailer lay in a garden near the village of Le Temple, with smoke rising from the charred forest nearby.

Finance Minister Roland Lescure said insurance companies had pledged to pay the housing costs for those displaced, "whether or not their home has been damaged".

France's fire has caused a "pyrocumulonimbus" -- a giant fire cloud that can create its own winds and packs lightning and sometimes tornadoes that can cause more blazes, according to France's national firefighters federation, who said the phenomenon had never before been seen in the country.

The weather office has forecast more hot weather in France from Tuesday, with temperatures than could soar to 40C and dry wind.

"All the conditions are in place for the fire to start up again," Philippe de Gonneville, the mayor of the village of Lege-Cap-Ferret, told AFP.

Fires have roared through 115,000 hectares in France since the start of the year, the authorities have said.

Roaring flames in the UNESCO-listed Fontainebleau forest outside Paris earlier this month devastated 2,000 hectares, in a rare such fire so far north.