LYON (AFP) - Six French climbers have died after a terrifying 250m fall from Mont Blanc, officials said on Wednesday, one of the worst accidents in a decade on Europe's highest mountain.

"We are certain that the climbers unfortunately died on the spot after a fall of around 250m," rescue police spokesman Frederic Labrunye told reporters.

The group of four men and one woman, aged between 27 and 45 and accompanied by a experienced guide, set out on Tuesday for the "Aiguille d'Argentiere" peak on the Mont Blanc massif, which stands at 3,902m.

The weather deteriorated rapidly and the accident likely happened some time around midday, authorities said.

Frantic rescue efforts began after the group failed to return to the refuge as planned but were severely hampered by bad weather.

Five bodies were found early on Wednesday, with the sixth discovered in a crevasse later in the day.

The accident came amid growing fears that Mont Blanc is increasingly becoming a tourist "free-for-all".

Agnes Robine, a prosecutor in the Alpine town of Chamonix, said the group members were in the second week of a two-week course to perfect their climbing skills. "These were not novices in terms of climbing," she said.

A judicial investigation has been launched, notably to see if anyone was at fault for the accident but at this stage it is "far too early to draw any conclusions," said the prosecutor.

The head of the refuge where the group was staying said the 42-year-old guide, identified only as Pierre, was "an experienced guide who did this trip every fortnight."

"They left at 4am in very good conditions," the refuge keeper, Fred Laurenzio, told AFP.

The accident, the single worst loss of life on the mountain in more than two years, caps a climbing season that has left 17 people dead or missing, according to local police official Georges-Francois Leclerc.

"This is not an especially deadly year," he told reporters.

Two Belgians were found dead on August 2 and six climbers died between July 15 and 30 - two Irish, two Finns, a German and a French person.

A US climber sparked outrage earlier this month when he tried to climb the mountain with his nine-year-old son and 11-year-old daughter and got caught in an avalanche.

The family escaped uninjured, but video footage of the incident in a spot known as the "Corridor of Death" caused an outcry when it was broadcast in the United States last month.

Laurenzio told AFP that on that particular peak "a small problem can very quickly become a big problem because you have to get down on your own steam and raise the alarm".

"On the Argentiere peak, there is no signal, it's a black hole for mobile rescue transmissions," he said.

The worst single death toll from an accident on Mont Blanc in recent times came in July 2012, when nine climbers - Swiss, German, British and Spanish - were killed in an avalanche.