Foreign rescue workers were allowed on the scene of a devastating earthquake in China as a strong aftershock sparked landslides Friday near the epicenter of the 7.9-magnitude temblor, again cutting off ravaged areas of central China.

Across the disaster zone, survivors still were being pulled alive from the rubble after four days buried, and the first foreign rescue workers arrived. Helicopters dropped leaflets into isolated mountain towns urging victims to cooperate to survive.

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An aftershock rattled parts of central Sichuan province Friday afternoon, the official Xinhua News Agency said, burying vehicles on the road repaired only Thursday to take supplies into the epicenter. The number of new casualties there was unknown. Xinhua said urgent repairs reopened the road and restored mobile phone links four hours later.

The U.S. Geological Survey said the latest tremor measured magnitude 5.5 and was centered 6.2 miles below the surface, a relatively shallow quake, like the initial disaster. The aftershock was one of the strongest among dozens that have shaken the area.

Education and housing officials took the rare move of fielding questions online from angry Chinese citizens over the many children among the official death toll, which the government said Friday had risen to about 22,069. Another 14,000 were still 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, and officials have said they expect the earthquake will eventually claim more than 50,000 lives. Jiang said the water supply situation was "extremely serious" in Sichuan, and not flowing at all in 20 cities and counties.

A day past what experts call the critical three-day window for finding buried survivors alive, rescuers pulled a nurse to safety who had been trapped for 96 hours in the debris of a clinic in Beichuan county, one of 17 people saved there, Xinhua reported.

Survivors were also being found elsewhere, with a man pulled from the wreckage of a fertilizer plant near Shifang city. But caring for the untold tens of thousands or more survivors across the earthquake zone was also 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, though there have so far been no outbreaks of disease.

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 from their home to the camp. He and his family were sleeping on donated bamboo mats and blankets that night.

Nearer the epicenter in the town of Yingxiu, helicopters dropped leaflets urging people to "unite together" and giving survival tips, such as not drinking 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 the winding mountain road to Beichuan, also near the epicenter, carrying backpacks and bags with food and medical supplies, on a quest for missing relatives. Military trucks and cranes edged around huge fallen boulders.

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 farther 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 $772 million for earthquake relief, according to the central bank, up sharply from $159 million two days ago.

China has also received $457 million in donated money and goods for rescue efforts, the Ministry of Civil Affairs said, 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.

Meanwhile, France's nuclear protection watchdog said the earthquake caused minor damages to nuclear facilities being dismantled but no apparent radioactive leaks.

Thierry Charles of the Institute for Radiological Protection and Nuclear Safety said Chinese nuclear authorities "reacted well" to the quake and immediately shut down nuclear sites for inspection.

He said Friday that Chinese colleagues have reported no radioactive leaks since the quake. He said "light damages" have been reported to older nuclear facilities that are being dismantled.

China's government has a research reactor, two nuclear fuel production sites and two nuclear weapons sites in Sichuan province.

The government said it had allocated a total $772 million for earthquake relief, according to the central bank, up sharply from $159 million two days ago.

China has also received $457 million in donated money and goods for rescue efforts, the Ministry of Civil Affairs said, 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.