China has found what it termed 50 "hazardous sources of radiation" due to last week's earthquake, a senior official said yesterday, although he insisted the situation was under control.

But Wu Xiaoqing, vice environment minister, said there had been no accidental releases of radiation.

"Thirty-five of the radiation sources have been recovered, and the location of another 15 has been confirmed, but they have not yet been recovered," Mr Wu told reporters in Beijing.

"Three are buried in rubble and another 12 are in dangerous buildings, which staff cannot go into," he added. "At present, tests from the scene show that there has yet to be an accidental release of radiation."

The disaster area is home to China's chief nuclear weapons research lab in Mianyang, as well as several secretive atomic sites, but no nuclear power stations.

In Washington, US State Department spokesman Tom Casey said he had no information to suggest that the radioactive sources identified by China were a hazard to people but he said such a possibility was always worrisome.

"Anytime you have nuclear material, or other potential hazardous material, out there, you want to make sure everything's done to be able to contain (it)," he said.

Eleven days after the 7.9 magnitude quake shook the mountainous province of Sichuan, hundreds of thousands of soldiers, relief workers and normal residents are now focused on reconstruction.

The known death toll from the quake already exceeds 55,000, but more bodies are expected to be found as the debris from the dozens of flattened cities, towns and villages is cleared.

A top provincial official said China would need to rebuild whole towns and villages from scratch to rehouse the millions displaced by the quake, a task that could take three years.

Some towns in the earthquake zone in the southwest of the country would need to be relocated altogether because the terrain is not safe, officials have said.

The rainy season, due within weeks, is adding urgency to their work. The government's main concern is that aftershocks and heavy rain could cause secondary disasters such as flashfloods and landslides.

"The rebuilding work faces difficulties in the region, where the mountains have been shaken loose in the earthquake and there have been more than 7,000 aftershocks," Li Chengyun, vice governor of Sichuan, told a news conference in Beijing.

Relief workers are also concerned that poor hygiene could cause disease outbreaks. Mr Li said this was a "peak period for outbreaks of diseases," describing the situation as very grim.

China has pleaded with the international community to provide millions of tents for the homeless. It will also send tonnes of heavy building equipment and supplies into the area.

"We will strive to provide safe, economical and convenient temporary housing for 98 per cent of the residents within the next month," Mr Li said.

In Chengdu, some volunteer relief convoys reported being held up by hungry residents, one at gunpoint. Two people were decapitated by helicopter blades, one at the Wolong panda reserve, sources in Sichuan and local reporters said.

UN Secretary-General Ban Ki-moon, who has been in Myanmar to urge the military junta to end restrictions on foreign aid workers, will visit China today to show solidarity with the Chinese people, the UN said, without giving details.

Premier Wen Jiabao, making his second visit to the disaster zone, visited hospitals and tents sheltering quake refugees yesterday in Mianyang and Beichuan county, one of the worst-hit areas.

In a temporary shelter for the Beichuan Middle School, which lost up to 1,000 students and teachers in the quake, Mr Wen tried to cheer up children, writing a line on the blackboard reading "Deep distress resurrects a nation."