Thousands of Chinese fled to the hills on Saturday amid fears a lake formed near the epicenter of this week's earthquake would burst its banks.
The water level at the lake formed after aftershocks blocked a river was rising rapidly in Beichuan and "may burst its bank at any time", the official Xinhua news agency said.
A paramilitary officer told Reuters the likelihood of the lake bursting its banks was "extremely big".
A witness said by telephone the military was evacuating everyone in Beichuan, even rescue workers.
A Reuters journalist fled an area near the Beichuan Middle School, which President Hu Jintao visited on Friday. Soldiers were talking on the radio saying "all retreat" and there was a lot of dust in the air. Troops were leaving fast.
China has said it expects the final death toll from Monday's 7.9 magnitude earthquake to exceed 50,000. About 4.8 million people have lost their homes and the days are numbered in which survivors can be found.
Cabinet spokesman Guo Weimin, taking a long pause to compose himself as he read from an updated casualty report at a news conference, put the death toll so far at 28,881.
Sichuan Vice-Governor Li Chengyun said more than 188,100 people have been injured and about 10,600 people remain buried under rubble. About 2.6 million tents are needed to shelter 4.8 million displaced residents, he added.
Hong Kong cable television said some 1.2 million people were also being evacuated in Qingchuan, 90 km (55 miles) northeast of Beichuan, as rising waters threatened to burst a lake's banks.
There has been growing concern about the safety of dams and reservoirs which have been weakened in the mountainous province of Sichuan, an area about the size of Spain.
A cable repair worker was killed on Saturday, five days after the original disaster, when hit by rocks as a moderate aftershock, one of hundreds, hit Lixian county.
Many survivors were also found, including a German tourist who was pulled from rubble in Wenchuan after being buried for 114 hours, Xinhua said.
A 69-year-old villager was one of 33 people rescued in Beichuan. He was buried for 119 hours. Troops evacuated 18 scientists trapped in a forest in nearby Mianzhu.
On Friday, soldiers pulled 2,538 people from rubble, only 165 of whom were still alive, the cabinet spokesman said, an indication hope of finding survivors was slim.
"Although the time for the best chance of rescue, the first 72 hours after an earthquake, has passed, saving lives remains the top priority of our work," President Hu told distraught survivors just over a week after a jubilant China celebrated the Olympic torch reaching the summit of Mount Everest.
Biggest since the communist revolution
Premier Wen Jiabao said the quake was "the biggest and most destructive" since before the Communist revolution of 1949 and the quick response had helped reduce casualties.
That compares even with the 1976 tremor in the northern city of Tangshan which killed up to 300,000 people.
And as the weather gets warmer, survivors were worried about hygiene and asking questions about their longer-term future.
"What we don't need now is more instant noodles," said truck driver Wang Jianhong in the city of Dujiangyan. "We want to know now what will happen with our lives."
In Sichuan and neighboring Chongqing, at least 17 reservoirs have been damaged, with some dams cracked or leaking water. Several are on the Min river, which tumbles through the worst-hit areas between the Tibetan plateau and the Sichuan plain.
The Lianhehua dam, built in the late 1950s northwest of Dujiangyan, showed cracks big enough to put a fist in.
"When the dam is in this shape, we cannot feel relaxed," said farmer Feng Binggui who has moved from his village below the dam into the hills.
China is also on precautionary alert against possible radiation leaks, the Ministry of Environmental Protection said. The country's chief nuclear weapons research lab is in Mianyang, along with several secret atomic sites, but there are no nuclear power stations.
China has sent 150,000 troops to the disaster area, but roads buckled by the quake and blocked by landslides have made it hard for supplies and rescuers to reach the worst-hit areas.
Offers of help have flooded in and foreign rescue teams from Japan, Russia, Taiwan, South Korea and Singapore have arrived. Donations topped 6 billion yuan ($857 million).