The raging blaze near the city of Bordeaux has gutted a number of homes and forced 10,000 residents to flee.
"It's an ogre, it's a monster," firefighter representative Gregory Allione told France's RTL Radio.
Strong winds and high temperatures are hampering the firefighting operation.
President Emmanuel Macron announced that Austria, Germany, Greece, Poland and Romania "are coming to aid" France in battling the blaze. "European solidarity at work!" he tweeted.
This summer France and a number of other European countries have seen a wave of deadly wildfires, triggered by record temperatures and droughts across the continent.
More than 1,000 deaths have been attributed to the heat in Portugal and Spain.