An aftershock brought new havoc to the earthquake-stricken region of China on Friday, as it struggled to bury some of its estimated 50,000 dead, dig out more survivors and help thousands of injured and homeless.

President Hu Jintao flew to the battered province of Sichuan and Premier Wen Jiabao says the quake damage could exceed that of the devastating 1976 tremor in the north-eastern city of Tangshan, which killed up to 300,000 people.

Mr Wen called on officials to ensure social stability as frustration and exhaustion grew among survivors, many of whom have lost everything and are living in tents or in the open.

'rising death toll'

China put the known death toll at just over 22,000 on Friday but has said it expects it to eventually exceed 50,000. About 4.8 million people have lost their homes.

Thousands of men, women and children were heading on foot for Mianyang, a city near the epicentre, saying they were abandoning their ruined villages for good.

Anger has focused on the state of school buildings, many of which crumpled in Monday's 7.9 magnitude quake, burying thousands of children and prompting the Housing Ministry to order an investigation.

Mr Hu and Mr Wen stressed that searching for survivors remained the top priority.

"We cannot talk about giving up too easily," Mr Wen says. "Life should go on. I believe people in the quake area can definitely build their hometowns even better with their own hands. That is also the biggest consolation for the dead."

The country is on precautionary alert against possible radiation leaks, according to a government website.

The disaster area is home to China's chief nuclear weapons research lab in Mianyang, as well as several secretive atomic sites, but no nuclear power stations.

Thousands of residents from Beichuan, one of the places worst hit, streamed away from the town carrying babies, bags and suitcases.

The town was a scene of devastation, with virtually every building either demolished or damaged beyond habitation.

To the south, in the village of Houzhuang, residents said they were coping on their own, aid and troops yet to reach them.

"We ate some corn, but now we are suffering from diarrhoea after drinking water from the ditch for two days," one says.

'after shock'

The aftershock, measuring 5.9 on the Richter scale, hit Lixian, to the west of the epicentre in Wenchuan, cutting newly repaired roads and telecommunications.

"A number of vehicles were buried in landslides. The casualties were not known," Xinhua news agency says, adding four of its reporters narrowly escaped death when a house collapsed.

China has mobilised 130,000 troops to the disaster area, but with buckled and blocked roads, supplies and rescuers have struggled to reach the worst-hit areas.

Offers of help have also flooded in. The first foreign rescue teams, from Japan, Russia, Taiwan, South Korea and Singapore have arrived in Sichuan province.

At China's request, the World Food Programme said it was sending enough ready-to-eat meals for 118,000 people.

UN Secretary-General Ban Ki-moon announced emergency funds of up to seven million dollars and said more would be available.

The United States said it had provided China with satellite images of earthquake-stricken areas, and would send two planeloads of relief for victims this weekend.

Xinhua says 33 people were dug out of the rubble in Beichuan still alive on Friday.

Peng Zhijun, 46, had eaten cigarettes and paper napkins and had drunk his urine to survive. A 50-year-old worker was rescued from a collapsed fertiliser plant after being trapped for 100 hours, a witness told Reuters.

Many survivors were on the move, desperately seeking food, shelter or medical treatment. Ning Feng told Reuters he had spent two hungry days limping out of the town of Yingxiu after deciding that waiting to be rescued could be more dangerous than risking landslides and exhaustion on the trek out.

"I had to do it on my own. Who was there to help me?" the 19-year-old painter of traditional Tibetan art said, as he stopped to rest on the winding mountain path.

'school collapse' 

In Dujiangyan, a school collapse buried 900 students. In Wufu, nearly every building in the village withstood the quake but for a primary school, whose collapse killed about 300.

"Our child wasn't killed by the earthquake. She and the others were killed by a derelict building. The officials knew it was unsafe," says Bi Kaiwei, whose daughter, 13, was killed.

Rescuers found two girls, one in a coma and the other dead, holding hands in the ruins of their school, Xinhua said.

Housing Minister Jiang Weixin said the schools had not been designed to withstand such a powerful earthquake, but added that corruption may have led to substandard construction.

"At this stage we cannot rule out the possibility that there has been shoddy work and inferior materials," Jiang told a news conference in Beijing.

There were also concerns about epidemics if the dead were not soon buried or cremated.

"A lot of tourists have been killed. We don't know how to deal with the bodies, some of which have been highly decomposed, but their relatives will come to look for them," an army officer in the badly hit Yinmugou resort in Pengzhou told Sichuan TV.

"I am really worried about epidemics," he says.

Hundreds of damaged dams have also raised fears of collapse and flooding of areas struggling to recover from the quake.