Beijing  -  A south-western Chinese town near the epicentre of Monday's devastating earthquake faced a growing risk of flood on Thursday, prompting troops to evacuate as many people as possible while they dug through rubble in the hope of rescuing some of the thousands of people still missing in the town.

A lake had formed on the Bai He river above Beichuan town after a landslide caused by the earthquake blocked the river, an official from the Sichuan Seismological Bureau told Deutsche Presse-Agentur dpa by telephone.

Because of the high risk of flooding, troops evacuated everyone to higher ground before returning to continue their rescue work in Beichuan, which was home to about
30,000 people, state media said.

The Sichuan official declined to respond when asked how the government planned to tackle the blockage on the Bai He.

Officials estimated that up to 5,000 died in Beichuan, where about 80 per cent of buildings collapsed in the old town and 60 per cent in the new town.

Minister of Water Resources Chen Lei on Thursday said that quake damage to rivers, dams, reservoirs and hydropower plants posed a "serious threat" of flooding in several areas of Sichuan and neighbouring provinces.

Most of the scores of reservoirs in Sichuan had sustained "significant but still unknown damage" during Monday's 7.8-magnitude quake, state media quoted Chen as saying.

"Inadequate management systems and poor data collection" meant that officials also did not know the full extent of damage to hydropower plants, the official Xinhua news agency quoted him as saying.

Flood control at damaged reservoirs, hydropower plants and dikes would be a crucial part of the government's relief work, aided by satellite imaging and aerial photography, he said.

More residents may have to be evacuated along rivers with a high flood risk, Chen said.

But the water ministry deemed the Zipingpu dam, close to the quake epicentre in Sichuan's Wenchuan county, "structurally stable and safe," the agency said.

Workers and engineers had rushed to build a temporary outlet from the reservoir behind the dam on Wednesday, after quake damage caused a build-up of water, cracked the dam wall, and destroyed buildings at the Zipingpu hydropower plant.

"If Zipingpu develops a serious safety problem, it could bring disaster to Dujiangyan city," where some 500,000 people live downstream along the Min River, the ministry said in a statement on its website on Wednesday.

Paratroopers who gave an initial report to state media on Wednesday from Sichuan's isolated Maoxian county said two more dams were at risk of breaking at hydropower stations that were "severely damaged" in the quake.

But the famous Dujiangyan irrigation system, which dates back more than 2,000 years, was reported safe.

There was also no significant impact from the quake on the massive Three Gorges dam and hydropower plant along the Yangtze River, reports said. (dpa)