By Alex Kirby BBC News Online environment correspondent

Scientists say more than 40 Himalayan lakes could soon overflow, imperilling tens of thousands of people.

They say the lakes are filling up because rising temperatures are melting the surrounding glaciers and snowfields that feed them.

Regional air temperatures are 1C higher than they were in the 1970s.

Work has started to lower the water level in one lake in Nepal.

The scientists work for the United Nations Environment Programme (Unep) and the International Centre for Integrated Mountain Development (Icimod).

Surendra Shrestha of Unep said: "Our findings indicate that 20 glacial lakes in Nepal and 24 in Bhutan have become potentially dangerous as a result of climate change.

"We have evidence that any one of these could, unless urgent action is taken, burst its banks in five to ten years' time.

"These are the ones we know about. Who knows how many others, elsewhere in the Himalayas and across the world, are in a similar critical state?"

Warning system

The lake where remedial work has begun is Tsho Rolpa, which researchers say is six times larger now than in the late 1950s. It was identified as critical by ground surveys and satellite images.

Downstream areas are at risk

Pradeep Mool of Icimod said: "A flood from this lake could cause serious damage down to the village of Tribeni, which is 108 km (67 miles) downstream, threatening about 10,000 people, thousands of livestock, agricultural land, bridges and other infrastructure."

A network of sensors and sirens has been built to link the lake to threatened villages. Engineers hope to lower the lake level by 30 m (95 feet).

Mr Shrestha said his team was working to help Nepal and Bhutan to identify potentially dangerous lakes, develop early warning systems and reduce the threats.

He said: "Some donor country governments are backing our efforts, but much more aid is needed. Solving this problem is going to be costly, because glacial lakes are situated in remote areas which are difficult to reach."

The UN has declared 2002 the International Year of the Mountains.

Dr Klaus Toepfer, executive director of Unep, said: "Mountains were once considered indomitable, unchanging and impregnable. But we are learning that they are as vulnerable as the world's oceans, grasslands and forests.

Dryer future

"It is not just the risk to human lives, agriculture and property that should worry us. Mountains are the world's water towers, feeding the rivers and lakes upon which all life depends.

Glaciers are in retreat

"If the glaciers continue to retreat at the rates being seen in places like the Himalayas, many rivers and freshwater systems could run dry."

Unep says there is evidence that glacial lake outburst floods (glofs, as they are known) have been happening more often over the last thirty years.

It says data from 49 Nepalese monitoring stations shows temperatures in the high Himalayas have been rising over that period by an average of 0.06C annually.

Rapid melting

The research into lakes at risk began in 1999, and is based on topographic maps, aerial photographs and images from Landsat, Spot and IRS satellites.

The scientists say that glaciers in Bhutan are retreating at 30-40m annually. One glacier, Tradkarding, which feeds the Tsho Rolpa lake, retreated 100m last year.

Scientists from Bhutan and Nepal worked with Unep and Icimod on the survey. An international conference on protecting the world's mountains is being held in late October in Bishkek, the capital of Kyrgyzstan.