At least six people were killed in flash floods in the southwestern Aude district of France after several months' worth of rain fell in just a few hours overnight, the local government prefect said on Monday.

Six helicopters were scrambled to help rescue people from the roofs of their homes, but bad weather was making operations difficult, the prefect of Aude, Alain Thirion, told BFM TV.

Around 250 firemen and 100 police were involved in the effort.

"We have people stranded on rooftops. We're going to have to use aircraft to evacuate them because we cannot reach them by boat given the force of the water. It's too dangerous," he said.

Among the worst hit areas were the villages of Conques-sur-Orbiel and Villardonnel, where waters rose as high as the first-floor windows of some houses. The Aude region is best known for its Medieval fort city of Carcassonne.

The flash floods struck without warning.

At least one victim was swept away by raging waters while sleeping, Thirion said. Television pictures showed rivers flowing furiously, having torn down trees and concrete power pylons, and swept away cars.

Water levels were expected to continue to rise in the area on Monday, with rain forecast until late in the morning. Schools in the area were closed and residents were told to stay at home.

Torrential rains are not uncommon in France at this time of year, but meteorologists have said exceptionally warm sea water along the Mediterranean coast may be intensifying the rains.

The year so far is France's hottest since 1900, according to the state meteorological service.