Chinese troops on Saturday eased pressure on a swelling "quake lake" that had threatened hundreds of thousands of people, with water gushing into a man-made sluice in an operation monitored by satellite.

A smaller quake lake at Majingxiang, also in the southwestern Sichuan province, burst its banks on Friday, witnesses said, shooting out bodies of people killed in the earthquake amid a torrent of gravel and rock that highlighted the threat posed by the huge dams of pent-up water.

More than 30 such lakes were created when the May 12 earthquake triggered landslides that blocked rivers, raising fears of secondary flooding disasters after the tremor, which killed more than 69,000 people.

Hundreds of troops were mobilised to dig a sluice channel to release water from Tangjiashan, the largest of the quake lakes, which has threatened hundreds of thousands of people downstream.

"At present there appears to be no danger of the dam wall collapsing," Zhou Hua, spokesman for the lake relief effort, told Reuters. "The situation is under control."

The backed-up lakes are merely one of the hazards and problems burdening millions of people living in tents nearly four weeks after the earthquake devastated their homes.

"What I'm most worried about is what happens next. We lost everything we had and we don't know what we'll get back," said Liu Shiyou, 57, a farmer from the nearby village of Chaping.

A construction worker described a mad scramble to safety when the Majingxiang lake they were trying to relieve unleashed a dangerous surge.

"We were releasing water through a man-made channel about two metres wide, but then it suddenly gave way and there was this roaring sound and we ran up the side of the hill," Chen Youfu said, while smoking a cigarette above the debris.

No one was reported injured in the incident, which took place in an isolated valley.

URGENCY

Chinese Premier Wen Jiabao was impatient to defuse the quake lake threat and return attention to the millions of people displaced by the earthquake, many of whom are living in crowded tent camps.

"The longer the delay, the greater the pressure created by the quake lakes, with safety threats multiplying and new hazards more likely to arise," Wen said near Mianyang, a threatened downstream city, on Friday.

More than 250,000 people have been evacuated in quake-ravaged areas of Beichuan, Mianyang and Jiangyou.

There are contingency plans to move up to 1.3 million to higher ground if the dam fully breaks.

The State Flood Control and Drought Relief Headquarters warned of heavy rain across China, including Sichuan, over the weekend. Deputy head Tian Yitang said the rain could put more pressure on quake-damaged waterworks.

"We should be on high alert for flood problems as the previous round of heavy rain had already raised the water levels in some rivers, and irrigation works and reservoirs in quake-hit areas were still not repaired," Tian said, according to the official Xinhua news agency.

The 220 million cubic metres of water caught in the Tangjiashan quake lake also threaten an oil pipeline that carries much of the needs of Sichuan and neighbouring Chongqing. Engineers have developed an emergency plan to use a temporary supply line if that pipeline ruptures, Xinhua reported.