Lacanau (AFP)

A military transport plane deployed by the French government on Saturday to help fight wildfires carried out its first firefighting mission, AFP journalists saw.

The A400M military aircraft, equipped with a dispensing kit that can release 20 tonnes of water or fire retardant, flew over a forest fire in Lacanau in southwest France, leaving a vivid orange plume in its wake.

According to the government, the plane's capability represents "the equivalent of three Canadair aircraft in terms of water volume."

The deployment of the aircraft had previously been scheduled for the end of the month due to preparations and testing.

But Prime Minister Sebastien Lecornu ordered its immediate use on Saturday due to the severity of the fires raging in southwest France.

The ministries of interior and defence had signed a joint agreement with Airbus to make available and equip, on an experimental basis, an A400M aircraft with an aerial firefighting kit to help France douse the blazes.

The threat posed by the major wildfires raging in the department of Gironde "remains very serious," said the top regional official, Sophie Brocas.

If weather conditions remain favourable, the night should be "conducive to firefighting operations," said Marc Vermeulen, head of the Gironde fire service.

About 57,000 people were evacuated from villages close to the city of Bordeaux on Friday after the advancing wildfire grew so intense that it generated a "firestorm," a rare weather phenomenon that Vermeulen said had never before been observed in France.