About 80,000 people have been evacuated downstream of an unstable earthquake-created dam threatening to collapse, while Chinese troops rushed to carve a trench to drain the water before it flooded the valley.
The threat of flooding from dozens of lakes swelling behind walls of mud and rubble that have plugged narrow valleys in parts of the disaster zone is a new worry for millions of survivors.
More than 30 villages were emptied yesterday and the people were being sent to camps like the one outside Jiangyou, where 12-15 people crammed into each of about 40 government-issued tents pitched on a hillside overlooking the river.
"We were told that so far it is the safest place for us to stay if the dam of the lake crashes," said Liu Yuhua, whose village of Huangshi was one of those emptied. "But we will have to move farther uphill if the situation turns out to be worse."
Troops used explosives to blow up tree stumps that were hampering heavy-duty excavators at the newly-formed Tangjiashan lake near the town of Beichuan, the official Xinhua News Agency reported.
The magnitude-7.9 quake that struck Sichuan province on May 12 sent a mass of dirt and rocks tumbling in the valley about two miles above the town in a spot not accessible by roads, plugging a river that is now forming the lake.
Elsewhere in the region, workers also used explosives to level some buildings that were left teetering by the quake - a further sign that officials have stopped rescue and recovery efforts in some places.
The number of deaths from the quake climbed towards an expected toll of 80,000 or more. China's Cabinet said yesterday that 67,183 people were confirmed killed, with 20,790 still missing.
Aftershocks continued to rattle the region, causing more damage and injuries and jangling the already-frayed nerves of survivors. Two temblors yesterday caused more than 420,000 houses to collapse in Qingchuan county, Xinhua reported. Sixty-three people were injured, including six who were critically hurt.
The US Geological Survey measured a magnitude-5.2 aftershock just after 4pm local time and one measuring 5.7 about half-an-hour later.
In a live broadcast, state television showed heavy earth-moving equipment, which had been air-lifted in, carving a 200-yard channel to drain the water from the Tangjiashan lake.
Hundreds of troops were working around the clock to dig the channel that would divert the rising waters before they breach the rubble wall. Officials fear the loose soil and debris could crumble easily if the water starts cascading over the top, sending a torrent down into the valley.
The lake was holding 34 billion gallons of water and was rising by more than 3ft every 24 hours, Xinhua reported.
Downstream, officials rushed to evacuate people in the path of potential floodwaters. Xinhua said emergency workers toiled into the night to try to get 80,000 people out. Another group of about 80,000 have already been evacuated the valley, it said.
Tangjiashan is the largest of some 35 lakes created by rubble blocking rivers in the quake zone. Some rising floodwaters have already swallowed villages, though only Tangjiashan was posing a current risk.
Adding to the urgency, thunderstorms were forecast for parts of Sichuan this week - a foretaste of the summer rainy season that accounts for more than 70% of the 24ins of rain that falls on the area each year.