JIANGYOU, China (AFP) - - China on Sunday declared three days of mourning and suspended the Olympic torch relay nearly a week after a massive quake struck the country's southwest, as the death toll continued to mount.

The announcements came after a powerful aftershock rattled devastated Sichuan province, killing at least three people and hampering China's efforts to help nearly five million homeless facing the threats of disease and floods.

From Monday, the government ordered the national flag to fly at half-mast for three days and called for three minutes of silence nationwide at 2:28 pm (0628 GMT) Monday, exactly one week after the massive quake struck.

During the period of national mourning, the Olympic torch relay -- eagerly awaited across China -- has been suspended, organisers of the Beijing summer Games announced.

Authorities on Sunday raised the number of confirmed deaths to 32,476 from the quake and revised upward the strength of the quake to 8.0 on the Richter scale. The US Geological Survey has put its strength at a magnitude of 7.9.

Sunday's aftershock, with a magnitude of 6.0, shook some of the worst-hit parts of Sichuan, killing at least three people in the town of Jiangyou, local government official Liao Boxun told AFP by phone.

People in the town, which was dotted with thousands of blue tents for quake homeless, said the aftershock had spread new fears among a population already traumatised.

"When the aftershock hit, mothers hugged their crying children and tried to comfort them, telling them everything would be OK," Dai Yong, who works in Jiangyou, told AFP.

The region has suffered at least 24 aftershocks of 5.0 or above on the Richter scale since Monday's quake, amid all-out efforts to rescue more than 9,500 people still buried under the rubble of collapsed structures.

Premier Wen Jiabao on Sunday urged scientists at the China Seismological Bureau to improve research on earthquakes and develop better warning systems, the Xinhua state news agency said.

Two more people were pulled to safety after improbably surviving six days under the chunks of concrete and other debris, but the pace of such miracle rescues slowed markedly.

Officials have estimated the final death toll could eventually top 50,000.

In a bid to help quake survivors, two US military planes packed with food, power generators and other goods flew into Sichuan province, the first aid that China has accepted from foreign armed forces.

Rescue experts from Japan, Russia, Singapore and South Korea, as well as Taiwan and Hong Kong, have been allowed in to help the effort.

Rain also compounded the misery for many of the estimated 4.8 million who have been left without homes.

Tens of thousands of people huddled in appalling conditions in the town of Maoxian under leaky makeshift rain covers, in dire need of food, medicine and shelter, the Sichuan News Online site reported.

"The people have gone from one disaster to another," the report said.

Flood fears also persisted, with the Sichuan government saying landslides triggered by Monday's earthquake had blocked rivers and streams in at least 21 places, causing water levels to swell dangerously behind the rubble.

China had already on Saturday evacuated thousands of people in devastated areas near the quake epicentre due to one such blocked river.

Worse is possibly to come, with China's national meteorological centre predicting torrential rains later this week in the quake disaster zone, warning they could trigger landslides.

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

The military, however, said that nuclear facilities in the region had been checked for signs of any damage and were confirmed safe.

Relief workers by Saturday had finally restored land connections with the worst-hit counties of Beichuan and Wenchuan, allowing the full horror of the quake to begin to emerge.

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.

Xinhua reported that five employees were killed and at least three pandas were missing at the world-renowned Wolong panda breeding centre, which was heavily damaged.

The news appeared to contradict early reports that more than 80 giant pandas at the centre were confirmed safe.