The official Xinhua News Agency said earlier that a lake in Beichuan county "may burst its bank at any time".
It said 46 seriously injured people were in "dire need of help" in Beichuan, where the water level was rising rapidly.
Xinhua did not give details but Hong Kong cable television said some 1.2 million people were being evacuated in Qingchuan, about 90km northeast of Beichuan.
A witness said by telephone the military was evacuating everyone in Beichuan, even rescue workers.
Residents left homes for higher ground, but a local disaster relief official said the water in Haizi lake, nestled between two mountains, was not rising very quickly.
Experts were studying how to release the excess from the lake, the relief official in Mianyang, who refused to give her name, said.
Earlier, Guo Weimin, a cabinet spokesman, said that the confirmed quake death toll had risen to 28,881.
The government had previously said at least 50,000 people were believed killed in the disaster.
Survivors were still being found under destroyed buildings five days after the quake.
A 52-year-old man buried in the ruins for 117 hours was pulled to safety in Beichuan, just after a German tourist was found in Wenchuan county, Xinhua reported.
The vast majority of survivors are rescued in the first 24 hours after a disaster, with the chances of survival dropping each day, said Irving "Jake" Jacoby of the University of California, San Diego.
Jacoby heads a medical assistance team that responded to a 1989 earthquake in California, Hurricane Katrina in 2005 and other disasters.
A person trapped but uninjured could survive a week or even 10 days, and in extreme circumstances two weeks or more, he said.