A climber trying to scale Mont Blanc may accidentally have caused a slab of ice to snap off high in the French Alps, sparking an avalanche that swept nine European climbers to their deaths, authorities said.

A dozen climbers were injured and two were still missing by nightfall.

As a sheet of snow and ice thundered down the steep slope, several other climbers managed to turn away from the slide in time, regional authorities in Haute-Savoie said.

Two climbers were rescued as emergency crews using dogs and helicopters scoured the churned-up, high-altitude area in a frantic search for the missing. Their quest, hampered by the possibility of further avalanches, was called off by night-time.

Three Britons, three Germans, two Spaniards and one Swiss climber were known to have died, the prefecture of the Haute-Savoie region said.

An initial report of four missing was lowered to two, and officials noted the numbers of those involved in the drama may vary because some climbers may have struck out on their own. A group of 28 were known to have left a mountain refuge for the ascent.

Close to 90 people were involved in the search.

Among the dozen injured was an American, the only known non-European. A seriously injured Swiss citizen was transported to a Swiss hospital.

Early summer storms apparently left behind heavy snow that combined with high winds to form dangerous overhanging conditions on some of the popular climbing routes around Mont Blanc, the highest mountain in western Europe. Regional authorities had warned climbers earlier this summer to be careful because of an unusually snowy spring.

The Mont Blanc massif is a popular area for climbers, hikers and tourists but a dangerous one, with dozens dying on it each year. Chamonix, a top center for climbing, hosted the first Winter Olympics in 1924.

Some of the climbers were with professional guides, others were climbing independently.

Police said they were alerted around 5.25am Thursday to the avalanche, which hit a group of climbers - people from Switzerland, Germany, Spain, France, Denmark and Serbia - who were some 4000 metres high on the north face of Mont Maudit, part of the Mont Blanc range.

A block of ice 40 centimetres thick broke off and slid down the slope, creating a mass of snow that was 2m deep and 100m long, according to a statement by the prefecture.

"The first elements that we have from testimony are that a climber could have set loose a sheet of ice, and that sheet then pulled down the group of climbers below. I should say the incline was very, very steep on this northern face," Col. Bertrand François of the Haute-Savoie police told reporters.

It was not immediately known if the climber lived or died.

According to recent tweets from climbers, high winds led to overhanging ice slabs forming on the slope. Several days ago Chamonix saw a monsoon-like downpour which turned to snow at 3000m high.

Jonas Moestrup from the western Danish city of Randers heard about the accident as he was on his way down from Mont Blanc.

"Three days ago, we ascended it (Mont Maudit). It was shocking to hear, it could easily have been us," he told the Danish news agency Ritzau by telephone. "It is scary and tragic."

Still, he noted the allure of those foreboding, majestic Alpine peaks.

"It's part of the thrill that something can go wrong," he told Ritzau.

French Interior Minister Manuel Valls flew over the site later Thursday, describing it as "a particularly spectacular block of ice." He said the climbers appeared to be an experienced group, and that the churned-up snow had made the search particularly difficult.

French investigators will examine the circumstances of the deaths.

- AP