French authorities set about draining a lake trapped beneath a glacier in the Mont Blanc Alpine range on Wednesday to protect thousands of people in the valley below from a flood.

Workers toiled at an altitude of 3,200 metres (10,500 feet), pouring hot water on the ice to make a hole through which they will push a pump and start tapping off the water that has accumulated below one of the glaciers.

Some 65,000 cubic metres (2.3 million cubic feet) of water have gathered in a cavity which is under immense pressure from the ice above.

They estimated that if the water burst out it could flood the nearby valley of Saint-Gervais in half an hour and "nearly 900 families could be affected," Saint-Gervais-les-Bains mayor Jean-Marc Peillex told AFP.

"Some people moved away after we announced the operation, perhaps because they don't trust the alert system. Others aren't too concerned," he said.

The area is also a popular spot for skiers and summer holidaymakers.

The pump, which can pass 50 cubic metres of water an hour, aims "to let out an initial jet of water... to avoid the pressure-cooker effect," Peillex said.

"These first cubic metres of water are the most important," said the head of the operation, Nicolas Karr of the mountain conservation service. "They will allow us to secure the site by lowering the pressure."

The pump is targeting the main cavity containing 25,000 cubic metres and could also drain other surrounding pockets -- but draining away the contents entirely will take weeks, the team said.

Workers hope that "by pumping at this spot they will be able also to collect water trapped in other cavities" linked to the main one, and drain it away, said glaciologist Christian Vincent of the state scientific research body CNRS.

Three more powerful pumps will be flown by helicopter to help with the job, due to last until October, operating 24 hours a day with a team of 15 workers sleeping on the mountain and looked after by mountain guides.

"A month from now it will be much more difficult. There's a risk it will start snowing and we would have to deal with the risk of an avalanche," said one guide, Alban Faure.

An eruption from a similar lake under the glacier in 1892 caused a torrential flood of water, rocks and earth which engulfed the valley and killed 175 people, almost wiping out the village of Bionnay.

"Even if there is a real risk of a catastrophe, it's years since we've been evacuated," said one resident of Bionnay, 44-year-old teaching assistant Marie-Claire Durand.

Another resident, Marguerite Legon, said she was worried about the flood threat. "I can't wait for it to be finished," she told AFP. "It has ruined my summer."