BEICHUAN, China -- Rescuers pulled survivors from the rubble Friday who had been buried for four days as a strong aftershock sparked landslides near the epicenter of this week's powerful earthquake.
The first foreign rescue workers since Monday's magnitude 7.9 temblor were allowed to the scene, and helicopters dropped leaflets urging people to "unite together" and providing survival tips. Officials have said the quake's final toll could reach 50,000.
A day past what experts call the critical three-day window for finding survivors, rescuers pulled a nurse to safety who had been trapped for 96 hours in the debris of a clinic in Beichuan county, Xinhua reported.
A call from the ruins of an apartment building drew a group of volunteers, who spent more than four hours using hands and spades to rescue a middle-aged woman. Brought to the surface, she could not speak and was handed over to medics.
"She had the will to live," said Xu Tao, one of the volunteers, a demobilized soldier and now an office worker in the eastern city of Tangshan. "I'm just exhausted."
About 10 people were pulled free Friday. Survivors also were being found elsewhere, with a man pulled from the wreckage of a fertilizer plant near Shifang city.
Dr. Irving "Jake" Jacoby of the University of California, San Diego, said the vast majority of people are rescued in the first 24 hours after a disaster, and that the chances of survival drop as each day passes.
If someone is trapped but is relatively uninjured, they could survive for a week or even 10 days, and in extreme circumstances two weeks or more.
Jacoby said people could live without food for a week, but water is needed to prevent dehydration. Jacoby heads a medical assistance team in San Diego that responded to the 1989 Loma Prieta earthquake in California, Hurricane Katrina and other disasters.
On Friday afternoon, an aftershock rattled parts of central Sichuan province, the official Xinhua News Agency said. A number of vehicles were buried on a road leading to the epicenter, and casualties were unknown. The U.S. Geological Survey said the latest tremor measured magnitude 5.5, one of the strongest among dozens that have shaken the area.
Education and housing officials, meanwhile, took the rare move of fielding questions online from angry Chinese citizens over the many children who died in the quake. The official death toll had risen to about 22,069 on Friday, and another 14,000 still were buried in Sichuan.
The government said it would investigate why so many school buildings collapsed in the quake -- destroying about 6,900 classrooms, not including the hardest-hit counties -- and severely punish anyone responsible for shoddy construction.
More than 4 million apartments and homes had been damaged or destroyed in Sichuan province, according to Housing Minister Jiang Weixin. Jiang 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.
Shifang's town square became a tented encampment holding 2,000 people and coordinator Li Yuanshao said there weren't enough tents. Many had walked from surrounding towns with few belongings.
"We brought almost nothing, only the clothes we are wearing," said Zhang Xinyong, a junior in high school who had walked several hours to the camp. They were sleeping on donated bamboo mats and blankets.
In the town of Yingxiu, helicopters dropped leaflets urging people to "unite together" and giving survival tips like not to drink dirty water. Power and water remained cut off, forcing dazed, exhausted locals to hike 40 yards up a steep hill to a spring to fetch water.
On another hillside, at least 80 corpses in plastic body bags were placed into a trench dug by soldiers.
Dozens of people trudged up a winding mountain road to Beichuan, carrying backpacks and bags with food and medical supplies, on a quest for missing relatives.
Liu Jingyong, a 43-year-old migrant worker searching for his cousin, traveled two days by bus and now foot just to get near his relative's home.
"I have not had any information from him," Liu said. "This is so hard on me."
One villager, Pan Guihui, stood on the side of the road with a vacant look on her face.
She and her husband had just hiked 13 hours with her 1-year-old child, father and two brothers away from their destroyed village further up the mountain. They had stayed in the rubble until rescue workers arrived and ordered them out because of fears of landslides.
"I have just been so frightened this whole time. I don't know what we are going to do," said Pan, 35. The only belongings the family had were some clothes and a little food, among hundreds camped along the road. "We've lost everything. There's nothing left of our village, nothing left of our home."
As she spoke, hundreds of soldiers marched by in long columns out of Beichuan, some carrying shovels.
In the city of Hanwang, Zhou Furen walked hours by foot -- borrowing the army green shoes she was wearing -- to a factory where her son had worked and remained missing.
"I've been coming here every day, sitting here in the early morning, waiting," she said, weeping. "He's been missing for more than three days now. But for my son I would come every day."
President Hu Jintao made his first trip to the disaster zone, rallying troops among the massive relief operation of some 130,000 soldiers and police.
"The challenge is still severe, the task is still arduous and the time is pressing," Hu was quoted as saying by Xinhua. "Quake relief work has entered into the most crucial phase. We must make every effort, race against time and overcome all difficulties to achieve the final victory of the relief efforts."
The first international rescue crews arrived in the disaster area, after China dropped its initial reluctance to accept foreign personnel. Japanese rescuers started work early Friday, and teams from Russia, Singapore and South Korea later joined operations, Xinhua reported.
It was the first time ever that China accepted outside professionals for domestic disaster relief, Foreign Ministry counselor Li Wenliang told Xinhua.
The government said it had allocated a total of $772 million for earthquake relief, according to the central bank's Web site, up sharply from $159 million two days ago.
China also has received $457 million in donated money and goods for rescue efforts, according to the Ministry of Civil Affairs, including $83 million from 19 countries and four international organizations.
Given the widespread destruction, AIR Worldwide -- a catastrophe risk modeling firm -- estimated losses to both insured and uninsured property would likely exceed $20 billion.
Associated Press writers Tini Tran in Hanwang, Anita Chang in Beijing and Stephanie Nano in New York contributed to this report.