A powerful aftershock rattled China's southwest early Sunday, hampering frantic efforts to find earthquake survivors and help nearly five million people facing the risk of disease and flood.

The 6.0-magnitude tremor shook some of the worst-affected parts in Sichuan province six days after China's worst natural disaster in a generation left an estimated 50,000 people dead.

China has suffered more than 20 aftershocks of 5.0 or above on the Richter scale since last Monday's initial 7.9-magnitude quake, amid all-out efforts to rescue more than 10,000 people buried under the rubble.

One survivor was pulled out Sunday after 139 hours under the debris of a flattened hospital in the ravaged Sichuan provincial town of Beichuan, state media said.

At least 63 more people were rescued alive Saturday, the state-run Xinhua news agency said, defying experts' warnings that survival chances greatly diminish three days after an earthquake.

But not all the rescue stories have happy endings. A man pulled alive from the rubble after 129 hours -- and whose leg had to be amputated by rescue teams -- died in a hospital Sunday of heart failure, state media said.

State television reported rescue crews in quake-ravaged region had gone on heightened alert following the overnight aftershock, fearful further strong tremors could hurt the teams.

It said rescuers were still trying to determine if there were any casualties from the aftershock.

"Rainfall and the aftershock have added difficulties to rescue efforts," it said, giving no details.

Xinhua said nuclear facilities had been checked for signs of any damage and "were confirmed safe and controllable."

Relief workers by Saturday had finally cleared through landslides and completed repairs to restore land connections with the worst-hit counties of Beichuan and Wenchuan.

The full horror is now emerging nearly a week after the earthquake, which damaged more than 15 million buildings in the remote, mountainous area that has escaped much of China's soaring economic growth.

Luo Hong, a 22-year-old woman who sells beer, learned that her 55-year-old father, Luo Zaiping, was killed at the coal mine where he worked.

"He worked hard his entire life. Originally he wasn't supposed to work Monday and then this happened," she said. "Now I want even more to work to help people who have survived."

The official People's Daily newspaper urged a nationwide "battle" against the disaster amid a rush by ordinary Chinese to volunteer.

"More than ever, people are aware that to win the battle against the devastating earthquake requires the contribution of the whole country," the communist party's flagship newspaper said in a commentary.

Major challenges lay ahead, with more than 4.8 million people left homeless.

Thousands of people were evacuated in Sichuan province Saturday over fears of floods due to a landslide that had blocked a river.

Rainfall also sent a mud-rock flow into a quake-hit town in Gansu province, which neighbours Sichuan, threatening to demolish more homes, Xinhua said.

Authorities were also racing to prevent the outbreak of disease, the risks heightened by the rotting carcasses of 12.5 million livestock and poultry.

"Combating epidemics is the most urgent and the biggest task facing us right now," Wei Chao'an, vice minister of agriculture, said Saturday.

The World Health Organisation said that the lack of safe drinking water or proper waste disposal along with cramped conditions in temporary shelters was "conducive" to outbreaks.

"Preventing communicable disease outbreaks is the key public health issue now facing the People's Republic of China," the UN body said in a statement.

President Hu Jintao, on a tour of Sichuan province, offered thanks to the foreign governments and organisations that provided help after the quake, which struck two months after unrest in Tibet set off an international furore that cast a shadow over the upcoming Olympic Games.

Teams from Japan, Russia, Singapore and South Korea, as well as Taiwan and Hong Kong, have been allowed in to help the effort, although other offers from elsewhere have been declined.