Some 900 hectares have been destroyed since the fire broke out on Wednesday afternoon in the rugged hills of the Aude department, about 100 kilometres southeast of Toulouse.
Commander Philippe Fabre of the Aude fire service told AFP there had been a “slight letup” early on Thursday, with firefighters able to halt the fire's advance.
But he said winds were expected to gust at up to 60 km/h later in the day, and efforts would be focused on “extinguishing the fiercest hotspots and preventing flare-ups in hard-to-reach areas”.
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No injuries have been reported and so far no homes have been destroyed, though 22 people were evacuated from their homes in the Val-de-Dagne municipality.
“Firefighters were able to stop the flames from reaching the houses but all the surrounding vegetation was destroyed. We had to evacuate the horses and donkeys,” said Christian Lacube, the municipality's mayor.
The cause of the blaze is not yet known, though authorities had warned of extremely dry conditions across much of southern France after weeks of drought.
“This fire is proving complicated because the areas are very difficult to access, there aren't any practicable roads,” Anne Laybourne, chief of staff for the government's top official in the region, told AFP.
Three tanker planes were dropping water on the flames, with two more on standby.
Some 500 firefighters have been called out to help keep the blaze at bay in Aude. Photo: Raymond Roig/AFP
Around 450 firefighters were also tackling a wildfire near Beziers along the Mediterranean coast that began on Wednesday, though officials said it was under control on Thursday after burning some 250 hectares.
Firefighters were working to stop the blaze reaching nearby villages.
“The fire is complicated because it is in an area difficult to access,” an Aude official said.The fire service said it planned to drop water bombs over the burning area, after dampening down the edges of the forest overnight.