China on Friday allowed the first foreign rescue teams to join the desperate search for earthquake survivors, but time was running out with an estimated 50,000 people already dead.

Across the devastated southwest province of Sichuan, troops, volunteers and medics clawed through mounds of shattered concrete that were once homes, schools and factories, seeking even the faintest signs of life.

Experts say the search-and-rescue operation is at a crucial phase, with the chances of finding survivors diminishing by the hour.

"The challenge is still severe, the task is still arduous and the time is pressing," President Hu Jintao said, quoted by state media, during a flight to Mianyang -- one of the cities hardest hit by the quake.

After earlier rebuffing foreign aid teams, China has now agreed to rescuers from Japan, Russia, Singapore and South Korea and has issued a nationwide appeal for tens of thousands of shovels, hammers and cranes.

"It is still within the critical period for saving lives and we won't give up if there is even the slightest hope of finding more survivors," Premier Wen Jiabao said, quoted by Xinhua.

The first foreign rescue team, from Japan, arrived early Friday and headed for a town where hundreds of families are reported buried under rubble, and a second Japanese team with sniffer dogs was due to arrive later in the day.

Xinhua quoted a foreign ministry official as saying it was the first time ever China's government had accepted foreign professionals for a domestic disaster rescue and relief operation.

Hu joined Wen in the quake zone. The Chinese premier jetted in shortly after the 7.9-magnitude quake struck Monday.

The epic scale of the quake -- which rattled buildings across China and in cities as far away as Thailand and Vietnam -- has become clearer after teams hiked their way through to remote towns cut off by landslides.

State television, quoting figures from national quake relief headquarters, said the government estimates more than 50,000 people have died.

Whole towns have been flattened, mountainsides sheared off, roads split in two, and countless thousands of buildings toppled or in danger of collapse.

Agonisingly, many of them have been schools, entire floors crushing down on each other and burying children in their classrooms.

Responding to public anger, China's housing ministry launched a probe into why so many schools crumbled, promising severe punishment for anyone found to have been responsible for shoddy construction.

Amidst the tragedy there have indeed been miracles -- an 11-year-old girl was hauled out of the rubble of her school in the flattened epicentre town of Yingxiu, 68 hours after the quake struck.

But increasingly, rescuers have been dragging out bloodied bodies, bringing a new problem of disposal in communities that have almost nothing left.

"I no longer have a wife and I lost my house," said Lui Wenbo, whose team has saved about 700 people in the town of Beichuan -- but could not reach his wife or her parents.

"It's hopeless," he said, pointing to the ground. "They're down here."

The military, which has been spearheading rescue efforts, has scaled up its deployment, sending in extra transport planes, helicopters and troops.

They have been air-dropping tens of thousands of food packets, clothes and blankets, clearing roads, repairing bridges, sifting through the wreckage and ferrying the injured to hospital.

Sniffer dogs were also sent in.

Rescue teams have also headed in from Taiwan -- which China considers to be part of its territory -- and Hong Kong.

Bai Licheng, a senior Communist Party official in Sichuan's Aba prefecture, warned of the risk of disease as bodies rot in the warmth.

"We are in urgent need of body bags," he said in Yingxiu. "Air-dropped food and drinking water are limited, and far from meeting the demand."

On Thursday afternoon, officials in Sichuan said the confirmed death toll there was 19,500 but several tens of thousands more were lying under debris.

Wen said the quake was the "most destructive" the country had known since the People's Republic of China was founded in 1949 -- more powerful than the 1976 Tangshan disaster, which claimed 240,000 lives.

PHOTO CAPTION:

Rescuers search for survivors

AFP