Chinese troops racing to drain an "earthquake lake" made substantial progress digging a diversion channel and have created emergency escape paths in case a mud and rock dam gives way.
The landslide-blocked river at Tangjiashan in southwest China is now the most pressing danger after an earthquake devastated the region on May 12.
Meanwhile, an official investigator pinpointed the poor design and construction of at least one of the many schools that collapsed during the 7.9 magnitude quake, killing thousands of children.
The death toll from the quake is more than 68,500 and is sure to rise with nearly 20,000 missing. Aftershocks have toppled 420,000 houses, most already uninhabitable.
A massive relief effort to provide food, tents and clothing for millions and rebuild houses and infrastructure is expected to take up to three years.
At the unstable Tangjiashan lake, hundreds of troops have removed more than a third of the earth for a channel intended to ease pressure from the rising waters in the mountainous province of Sichuan, an official spokesman said on Friday.
"The work on the blocked lake is going smoothly and at this pace it should be completed soon," said Zhou Hua, an official from nearby Mianyang city involved in the drainage effort.
Zhou declined to say when the operation was likely to finish. But up to 190,000 residents downstream had moved to higher ground - usually hillsides close to where they were living before - to avoid a surge of water if the blockage suddenly gave way.
"At this stage, the situation is under control, but we've set in place this contingency plan to minimise any possible damage."
Xinhua news agency said the water level was nearly 23 metres (75 feet) below the lowest point of the barrier, which experts have said could give way quickly once breached. Troops have also built escape paths in the event that happens, Xinhua said.
Post-quake reconstruction work has only just begun, and tens of thousands of survivors are now threatened by more than 30 quake lakes, formed by landslides, that could break through the natural dams, flooding downstream towns and reservoirs.
