Chinese troops began easing pressure on a dangerously swollen "quake lake" on Saturday, with water gushing into a man-made sluice in an operation monitored by satellite.
Hundreds of troops had been mobilised to dig a sluice channel to release some of the water in the lake that is threatening hundreds of thousands of people downstream in southwestern Sichuan province.
The Tangjiashan lake is the largest of more than 30 such dams formed 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.
The draining of the 220 million cubic metres of pent-up water was earlier delayed while troops rushed to expand the channel, worried the dam wall of rock and mud could give way and unleash a dangerous surge, Xinhua news agency reported.
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, according to state television news.
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.
Tian Yitang, deputy head of the headquarters, 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 Xinhua.
The Tangjiashan quake lake also threatens 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.