At least 10 hikers have died in southern Italy after the level of a raging white-water creek in a deep mountain gorge swelled suddenly after heavy rainfall.

Key points: Rescue teams are unsure how many people were hiking the gorge at the time

Rescue teams are unsure how many people were hiking the gorge at the time Raganello Gorge is a popular aquatic trekking spot in the Pollino National Park

Raganello Gorge is a popular aquatic trekking spot in the Pollino National Park At its most narrow point, it is just meters wide and some 400 metres tall

Officials said 23 people were rescued from the flash flood in the Calabria region, five of whom were hospitalised.

They included a 10-year-old boy who was being treated for hypothermia.

"This gorge filled up with water in a really short space of time and these people were catapulted out like bullets," Carlo Tansi, head of the civil protection department in Calabria, said.

"They ended up some three kilometres down the valley."

"It is really difficult terrain, filled with obstacles because of the (geological) formation of the area," said Eugenio Facciolla, the chief prosecutor of the provincial capital, Cosenza.

It was not clear how many people were missing because not all had entered the gorge with official guides and registered.

"The problem is we don't know how many people were knocked over by this flood," Mr Tansi told Sky TG24.

"This is a split in the terrain that is very tight and high."

In some places the Raganello creek, a popular aquatic trekking spot in the Pollino National Park, is at the bottom of a narrow, one-kilometre-deep gorge in the mountain.

At its most narrow point, it is just metres wide and some 400 metres tall.

Rescue teams used ropes to descend the sides of the mountain to reach the site.

Images on national television showed helmeted mountain rescue squads rushing from the nearest town, Civita, to reach the gorge, a popular tourist attraction in summer.

Spotlights were brought to the area so the search could continue during the night.

All of the victims were believed to be Italian tourists.

Reuters/AP