Flash flooding has killed at least 13 people in France across the southwestern region of Aude, after several months worth of rain fell in the space of a few hours overnight.

Five others are seriously injured and one person is missing, a spokesperson for the French Interior Ministry said.

Frederic de Lanouvelle said that the death toll could still rise, even as rain begins to ease.

Four deaths were clustered around the town of Villegailhenc and nine were from around one town, Trebes.

Alain Thirion, the prefect of the region, said some of the victims appear to have been swept away by floodwaters. In the town of Conques-sur-Orbiel, the river rose by more than six meters (20 feet).

An elderly nun died after floodwaters swept through a nunnery in Villardonnel, according to The Local.

The water rose as high as the first floor windows of some houses.

Floodwaters were in some cases too powerful for the emergency services to get through, even on boats, Mr Thirion added.

Six helicopters were sent to rescue people from the roofs of their homes but bad weather made operations difficult.

Vigicrues, the French agency that monitors rivers for flood risks, says water levels are higher than at any time since 1891.

A car is damaged in southern France (AP)

The floods struck without warning and at least one victim was swept away in their sleep, officials said.

Some people were helicoptered to safety overnight from the roofs of their homes, as water swept through roads and streams in the region swelled to become torrents.

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

An expected French government reshuffle was postponed because of the deadly floods.

Emmanuel Macron, the country's president, will travel to the flood hit area "as soon as possible" according to a French presidency official.

A date for the trip has not been set yet because Mr Macron does not want to disrupt emergency response work.

Television images and videos on social media showed waters coursing through towns and villages, with cars stranded in the floodwaters.

Ines Siguet posted a photo online of a road in Villegailhenc, where a powerful surge of muddy water and debris ripped out a bridge.

"There's nothing left, there's just a hole," she said. "It was very violent."

Schools were closed and authorities urged people to stay home.

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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.