China says more than 50,000 people are likely to have been killed in its devastating earthquake, as authorities begin digging mass graves for the dead.

* Death toll rises sharply
* Time running out for survivors
* Foreign aid workers allowed in
* Burial pits prepared
* New threat from damaged reservoirs

Time is running out to save those still trapped in the ruins of countless towns and villages across the country's south-west, four days after the 7.9-magnitude quake shook hundreds of homes, schools and factories to the ground.

"The deaths are estimated to be over 50,000," state television said, citing figures from the national quake relief headquarters.

The epic scale of Monday's quake is becoming clearer as teams hike into the remote epicentre in Sichuan province, where whole towns were levelled.

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"If there are some survivors under such conditions, it would be a matter of luck, or a miracle," said Zhang Zhoushu, vice director of the state-run China Earthquake Disaster Prevention Centre.

Yet amidst the tragedy, miracles did occur.

In Yingxiu, where three-quarters of the 10,000-strong population died, rescue workers pulled an 11-year-old girl from the rubble alive, 68 hours after the quake demolished her school.

Foreign aid accepted

Rescuers were sifting through the debris when they heard a voice.

"It's wonderful, she's alive!" a delighted onlooker shouted as the girl was pulled out on a stretcher and given a small cup of water.

Early on Friday, Beijing - which had initially rebuffed foreign offers of aid and assistance - said it had accepted help fromrescue teams from Russia, South Korea and Singapore.

A group of 31 Japanese search-and-rescue specialists have already arrived in Sichuan, but experts have warned the likelihood of finding survivors diminishes with each hour that passes.

"Most people are saved in the first three or four days," Willie McMartin, director of British-based charity International Rescue Corps, said in Hong Kong, where his team is trying to get permission to enter China.

"People can survive up to 15 days, but that is when you are talking about miracles, and miracles do not happen very often."

Thousands entombed

A planeload of aid from Taiwan's Red Cross Society arrived late on Thursday in Sichuan, where officials upped the confirmed death toll to more than 19,500, with tens of thousands more missing or entombed in debris.

As the military ramped up its rescue efforts with more troops and aircraft, a new threat emerged from creaking dams and reservoirs shaken by the quake.

State-run television said authorities had found "dangerous situations" at more than 400 reservoirs - two of them major - across five provinces.

Underlining the desperate efforts, China launched a mass public appeal for thousands of shovels, hammers and cranes, saying some rescuers were having to shift huge concrete slabs by hand to get to survivors.

"As long as there is only a glimmer of hope we will never give up," vowed China's vice health minister Gao Qiang.

Premier Wen Jiabao ordered another 30,000 troops and 90 helicopters to the area to bolster operations, while the military began its first major air drops of tens of thousands of food packets, clothes and blankets.

Military air drops

Sniffer dogs were also sent in to help look for survivors, as millions left homeless by the disaster spent their fourth night sheltering under tarpaulins in the open air.

"We must use all our forces, and save lives at whatever cost. Life is the most precious thing," Mr Wen told a meeting at quake relief headquarters.

But while many are still hoping for the best, officials have also begun preparing for the worst, digging a series of burial pits to dispose of the dead before disease spreads.

"There are still bodies in the hills and pits are being dug to bury them," said Zhao Xiaoli, a nurse in the ruined town of Hanwang. "There's no way to bring them down. It's too dangerous."

In Luoshui town in a quake-ravaged area, troops used a mechanical shovel to dig a pit on a hilltop. Two bodies wrapped in white sheets lay beside it. Down the hill sat four mounds of lime.

Across the quake zone in Dujiangyan, troops wearing face masks collected corpses and loaded them onto a flatbed truck, while thick black smoke streamed from the twin smokestacks of the town's crematorium.

Health officials said there had been no outbreaks so far, but health workers are rushing to inoculate survivors against disease, and supply them with clean drinking water to stave off the threat of epidemics.