The U.S. Geological Survey measured today's aftershock at magnitude 6. It was the second to hit in two days. The most recent one shook the area for 45 seconds, causing people to run into the streets.
Earlier, thousands of earthquake victims fled areas near the epicenter after police said rescue officials are worried that water could inundate nearby towns. The river is reportedly jammed up by a landslide caused by Monday's 7.9-magnitude earthquake.
Authorities say the confirmed death toll has risen to nearly 29,000, but the government has said at least 50,000 people are believed to have been 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 Dr. Irving "Jake" Jacoby of the University of California, San Diego, who 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.
Continuing aftershocks made digging through unstable buildings dangerous. On Friday afternoon, an aftershock rattled parts of Sichuan, burying vehicles on a road leading to the epicenter, Xinhua said.
Rescue teams from South Korea, Singapore and Russia got to work Saturday. They joined a Japanese specialist group, which was the first international rescue crew to arrive in the disaster area after China dropped its initial reluctance to accept foreign personnel.
A U.S. Air Force cargo plane loaded with tents, lanterns and 15,000 meals left Hawaii on Saturday, the first aid flight from the United States to help in Sichuan province. Another Air Force delivery was to fly in from Alaska.
The United Nations announced a grant of up to $7 million from its Central Emergency Response Fund, to be used by U.N. agencies and programs.
The government has not given a figure for the number of people left homeless, but Housing Minister Jiang Weixin said more than 4 million apartments and homes were damaged or destroyed in Sichuan province. He said the water supply situation was "extremely serious" in Sichuan, and not flowing at all in 20 cities and counties.
Caring for the untold tens of thousands or more survivors across the earthquake zone was stretching government resources.
Just north of the provincial capital of Chengdu, the town square in Shifang had become a tent camp for 2,000 people, and coordinator Li Yuanshao reported a lack of tents. Many people walked in from surrounding towns with few belongings.
"We brought almost nothing, only the clothes we are wearing," said Zhang Xinyong, a high school junior who walked several hours to the camp.
The Ministry of Health said there had been no major outbreaks of epidemics or other public health hazards in the earthquake area, according to Xinhua. By late Friday, hospitals in Sichuan had received 116,460 patients, including nearly 16,000 severely injured.