More than 50,000 people were feared dead in the devastating earthquake in southwest China as hopes of finding survivors faded on Thursday, with thousands still buried under the debris, three days after the worst quake in three decades hit the country.

Quoting the rescue headquarters of the State Council or the Cabinet, official Xinhua news agency said that over 50,000 were feared killed in the 7.8 magnitude quake that pulverized areas near the epicentre in southwest Sichuan province.

It said that in Sichuan alone, the death toll was 19,509.

China stepped up relief and rescue operations to cope with the disaster, pouring in more troops, rescuers, food and medical supplies to tens of thousands affected in areas, several reduced to rubble.

Nearly 130,000 troops raced against time to extricate victims from the debris in towns flattened by the quake.

Military transporters and helicopters made 300 flights to transport or airdrop rescuers and relief supplies. But hopes of finding more survivors slimmed down.

With blocked and damaged roads leading to areas near the epicentre being speedily cleared, rescuers were moving equipment needed for disaster relief.

In the front-line of rescue and relief efforts ever since the quake unleashed massive destruction, Chinese Premier Wen Jiabao moved from one affected area to another, speaking comforting and assuring words to people desperate for relief.

"See a large number of soldiers are coming. Food, water and tents will also come soon," he told children at Qingshan County, while encouraging local residents to "rise from sorrow, help each other and rebuild home."

"Within 72 hours after the disaster is the critical period. Generally, the sooner the rescue of the buried, the better," the chief engineer of Shijiazhuang Bureau of Seismology, Liang Guiping, told the state TV, as time was running out.

Amidst slimming hopes of pulling out survivors, there were also moments of relief as a 22-year old woman said, "thank you, thank you" to rescuers after she was taken out of the rubble of a hospital in Dujiangyan.

There were no reports of diseases among refugees who were being immunised, Gao Qiang, Deputy Health Minister said on the state-run CCTV. "We will try to achieve the goal of 'no big epidemic' after a great disaster."

In a rare appeal, the Chinese government asked the public to lend rescue equipment ranging from hammers, shovels and demolition tools to cranes and rubber boats and life detectors to help in rescue operations. Having to tackle a large scale devastation, many armed police and rescue workers had to remove cement slabs with bare hands to save the victims, news footage showed.