Dozens of new lakes were formed by the 12 May earthquake
Troops in China are trying to clear a river blocked during the recent earthquake, amid fears millions could be at risk of flooding.
About 1,800 soldiers took explosives to the newly formed "quake lake" to try to blast away debris, state media said.
The official death toll from the earthquake rose to 65,080 on Monday, with another 23,150 missing.
Safety concerns in the region remain high. On Sunday six people died from the effects of a strong aftershock.
The Chinese authorities say nearly 300,000 homes were wrecked in the aftershock, and more than 1,000 people injured.
The tremor was the most destructive yet of thousands of aftershocks in the last two weeks.
Dozens of lakes were formed when landslides triggered by the massive 12 May earthquake blocked rivers.
On Sunday, Vice Minister of Water Resources E Jingping said that the lakes were "under control" but described the situation as grim.
Heavy rain forecast for the next few days could cause the barriers blocking the lakes to burst and flood nearby areas, he told a news conference.
Of particular concern is the Tangjiashan lake, 2 miles (3km) from Beichuan town, which has been rising fast.
Water levels now stand only 29 metres below the level of the barrier, Xinhua said.
"The lake is now holding more than 128 million cubic metres of water and may cause a devastating flood if the barrier bursts," it said.
On Monday, troops carrying 10kg of explosives each arrived at the lake to begin blasting work.
Thousands of people immediately downstream from the lake have already been evacuated.
Up to Sunday 25 May:
62,664 people dead
23,775 still missing
More than 5.4 million homeless
638,305 rescued and resettled
More than 8,000 aftershocks, biggest 6.0
69 dams faced danger of collapse, 310 in dangerous state and another 1,424 facing moderate risks
Source: Chinese government
There is also continuing concern over the state of dams in the region.
E Jingping said 69 dams were at one time in danger of bursting, although steps have been taken to reduce the risk.
More than 300 more were affected by the earthquake, he added, saying many had been drained or had their water levels reduced.
Some five million buildings have collapsed across the region and around the same number of people are homeless, officials say.
Many people are living in tents or temporary shelters. China has appealed for international aid for the survivors.
Officials fear that the bad weather forecast for the next few days could hamper relief efforts.
A BBC correspondent in China, Dan Griffiths, says such conditions may thwart hopes to complete reconstruction within three years.
Troops gather to start work on lakes