BEICHUAN, China - China struggled to bury its dead and help tens of thousands of injured and homeless on Friday when a powerful aftershock brought new havoc four days after an earthquake thought to have killed more than 50,000.

|PIC1|President Hu Jintao flew to the battered province of Sichuan and Premier Wen Jiabao said the quake damage could exceed the devastating 1976 tremor in the north-eastern city of Tangshan that killed up to 300,000 people.

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

China put the death toll at 21,500 on Friday but has said it expects it to exceed 50,000.

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

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

"If only there is the slightest hope, we will spare no effort. If only there is one survivor in the debris, we will never give up," Wen said over the debris of a collapsed school where hundreds were buried.

Thousands of residents from Beichuan, one of the areas worst hit by Monday's 7.9 magnitude quake, streamed down the road away from the town, carrying babies, bags and suitcases as they left in search of shelter.

A body lay covered on a makeshift stretcher by the side of the road, abandoned by someone unable to carry it further. Rocks the size of cars lay on the surface, evidence of landslides triggered by the quake.

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," a resident surnamed Liu said.

"Now we've been trying to get things out of the debris to use, like clothes, but we're very frightened that there will be another earthquake, so we have to be very careful," he said.

BUCKLED ROADS, LANDSLIDES

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

"A number of vehicles were buried in landslides. The casualties were not known," Xinhua news agency said.

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

Neighbouring areas have also suffered, with more than 50,000 made homeless in one county of Gansu province to the north, Xinhua said.

But there were still small victories.

Rescuers saved a child from the debris of a school in Beichuan 80 hours after the quake struck. They said they could hear weak calls for help from amid the rubble, Xinhua said.

Three others in Beichuan were rescued on Friday, two in the remains of an office building and one in a collapsed hospital.

And 483 children and teachers escaped unscathed from a wrecked school in Beichuan.

Many raised questions about school construction.

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," said Bi Kaiwei, whose daughter, 13, was killed.

Two girls held hands in the ruins of their school promising not to give up hope. "When rescuers found them, one in a coma and the other dead, their hands were still clenched together," Xinhua said.

Housing Minister Jiang Weixin said the schools weren't designed to withstand such a powerful earthquake, but added corruption was a possible cause.

"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.

DAM THREAT

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

"We are in urgent need of body bags," Bai Licheng, a Communist Party official in Sichuan's Yingxiu town, told Xinhua.

Bodies were lined up along the riverbank in the town, where more than 3,000 soldiers were searching for survivors.

The Ministry of Health issued a notice ordering bodies to be cleaned where they were found and buried as soon as possible, far from water sources and downwind from populated areas.

Hundreds of damaged dams have also raised fears of collapse or flooding that could inundate towns and cities that are already struggling to recover from the quake.

China has asked the United States for satellite images to help locate victims and identify damaged infrastructure.

In Sichuan and neighbouring Chongqing, reservoirs have been damaged, some dams have cracked or are leaking water, and officials have warned the full extent of the hazard was as yet unclear.