CHAMONIX, France -- A climber trying to scale Mont Blanc accidently caused a slab of ice to snap off Thursday high in the French Alps, sparking an avalanche that swept nine European climbers to their deaths, authorities said. Eleven other climbers were hospitalized and at least four were still unaccounted for.

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 other 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 before nightfall.

Three Britons, three Germans and two Spaniards were among the dead, their governments confirmed. The other victim was from Switzerland, according to the gendarme service in the French mountain town of Chamonix.

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.

The gendarme service said it was alerted around 5:25 a.m. Thursday to the avalanche, which hit a group of climbers -- ones from Switzerland, Germany, Spain, France, Denmark and Serbia -- some 4,000 meters (13,100 feet) high on the north face of Mont Maudit, part of the Mont Blanc range.

A block of ice some 40 centimeters (16 inches) thick broke off and slid down the slope, creating a mass of snow that was 2 meters (6-foot) deep and 50 meters (160 feet) long.

"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 that the incline was very, very steep on this northern face," Col. Bertrand Francois of the Haute-Savoie gendarme service 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 3,000 meters (9,850 feet) 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.

A spokesperson for the British Foreign Office said the British ambassador to Paris was heading to Chamonix.

French investigators will examine the circumstances of the deaths.

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Charlton reported from Paris. John Heilprin in Geneva, Jan M. Olsen in Copenhagen, Sylvia Hui in London, David Rising in Berlin and Ciaran Giles in Madrid contributed to this report.