President Emmanuel Macron on Monday urged firefighters battling a huge blaze in southwest France to keep up their strength for some gruelling weeks ahead.

"The coming weeks will be tough and we have to hold out," he said as he visited a crisis centre in the nearby winemaking capital of Bordeaux.

A huge forest fire between the city and the sea has since Wednesday ravaged 42,000 hectares of forest -- an area larger than the US city of Detroit -- and destroyed the homes of some 240 families.

Macron pledged the government's support in the national response to one of the largest fires in France since World War II.

"We will be there, and together we are going to win this battle, and we will also be there over the long term," he said.