Chinese soldiers were working non-stop to dig a giant sluice to ease pressure on a swelling "quake lake", with plans to evacuate hundreds of thousands of people to avert a new disaster, state media said.
China put the death toll from the earthquake that struck Sichuan province on May 12th at 67,183, with the figure certain to rise with 20,790 listed as missing.
Nearly 362,000 people were injured.
Soldiers and police trekked to the Tangjiashan lake carrying dynamite ready to blast the mud and rubble blocking the flow of water from a river and creating the largest of 35 quake lakes formed when landslides triggered by the massive tremor blocked rivers.
Some 30,000 people living below the lake in and around Beichuan in the mountainous southwestern province have been evacuated as a precaution.
In Mianyang, 150,000 people will have been evacuated by midnight in line with a contingency plan should one third of the lake's 300 million cubic metres of water burst the dam, Xinhua news agency said.
"It's better for them to complain about the trouble that the evacuation would bring than to shed tears after the possible danger," Liu Ning, an official with the Ministry of Water Resources, was quoted as saying.
The lake had risen to 725.3 metres (2,380 feet) on Monday, only 26 metres below the lowest part of the barrier, he said.
By Monday night, around 600 engineers and soldiers had gathered at the landslip and were taking turns to work through the night with bulldozers dropped by helicopters into the area.
The troops were expected to finish the sluice by June 5th and would not discharge floodwater in the coming days even though the water level rose another 1.79 metres on Tuesday and authorities were preparing for the worst, Xinhua said.
Dangers at 28 quake lakes had yet to be defused and a total of 379 damaged reservoirs were still "high risk", Xinhua said, amid continuing aftershocks.
The most powerful of thousands of aftershocks killed at least eight people on Sunday, hampering relief efforts and terrifying quake survivors anew.
Aftershocks continued today, state television reported, with one measuring 5.4 on the Richter scale recorded in the quake-ravaged area of Qingchuan.
Another, measured at 5.7, rocked Ningqiang county in neighbouring Shaanxi province less than 40 minutes later. No casualties had been reported.
Mianyang, which includes Beichuan and where more than 16,000 people have died in the quake, replaced its mayor on Monday, but it was unclear if it was related to any dereliction of duty in relief work.
Chinese Premier Wen Jiabao said today that relief work would shift to reconstruction for the 15 million people displaced by the quake.
The massive relief effort, which involves food, tents and clothing for millions, as well as reconstructing housing and getting help to isolated villages, is expected to take up to three years.
The biggest appeal is for tents and pre-fabricated housing units for millions of homeless people as the weather turns warmer and wetter just at the start of the normally sweltering summer, risking the spread of disease.
"Temperature in the tents will be between 31 and 37 degrees Celsius in the next two days, so ventilation and cooling measures must be kept in mind," Xinhua said.
Health officials said they were confident large-scale epidemics could be prevented.