China rushed Friday to remove radioactive and chemical materials sitting downstream from a "quake lake" that threatens to burst and send torrents of water into heavily populated areas.

Nearly 100 unidentified radioactive sources were ordered to be removed by Friday evening from the path of the potential torrent of water, state press reported, citing the nation's environmental protection bureau.

"Moving those radioactive sources has become a top, urgent priority," the Beijing Times quoted Ma Ning, a senior regional official at the bureau, as saying.

The directive to move the radioactive material came as authorities were already working to relocate about 5,000 tonnes of dangerous chemicals that were downstream of the lake at Tangjiashan.

Dealing with the "quake lake" has become one of the key challenges in the aftermath of the May 12 earthquake that devastated large tracts of mountainous Sichuan province, killing more than 68,800 people.

The lake was created when landslides triggered by the quake created a dam across a river in a valley.

Highlighting the sense of alarm, the official Xinhua news agency reported Friday that 1.3 million people had been ordered to evacuate, but residents and a local government spokesman immediately denied there was any such order.

Helicopters have been used to airlift supplies to hundreds of soldiers working desperately to create a channel that can drain the lake, which contains enough water to fill over 50,000 Olympic-size swimming pools.

After three days of non-stop efforts, the soldiers had dug a 50-metre (164-foot) wide channel 300 metres long, but despite the frantic pace the work would not be completed until next Thursday, the state-run China Daily reported.

More than a million people risk being affected if the Tanjiashan lake empties onto towns and villages downstream.

Nearly 170,000 people have already been evacuated while many others have been doing regular drills to move quickly to higher ground in the event that the lakes does burst.

However, it was not the only area of Sichuan at risk. There were 33 other lakes created by the quake, 28 of which were at risk of bursting, the official Xinhua news agency reported.

Other unexpected dangers also continued to arise amid the massive task of looking after the 15 million people made homeless in the quake.

Gas from a chemical fire in Leigu town, near the epicentre of the quake, poisoned four people and forced more than 800 to evacuate on Thursday, Xinhua reported, citing a local official.

The fire occurred when bleach powder, used as a disinfectant, self-ignited when it reacted with leaked rainwater, said Song Ming, Communist Party secretary for Beichuan county, one of the worst-hit areas.

The dense chlorine gas poisoned two rescue soldiers and two medical workers, who were taken to hospital, according to Xinhua.

No one was available at the environmental protection bureau on Friday to comment on the report about the radioactive sources that were being cleared.

But previous reports in the state press said these sources could emanate from machines used to test defects in the construction of bridges or boats, or from X-ray machines.

There were also several nuclear installations not used for electricity generation in areas near the epicentre of the quake, according to the Institute for Radiological Protection and Nuclear Safety (IRSN) in France.

These included a manufacturing site for nuclear weapons, as well as a nuclear reactor.

The government said last week that nuclear facilities and radioactive sites in the Sichuan province were "safe and controllable."

Amid the tragedy, the quake has also caused a wave of international sympathy, helping to strengthen ties not least with China's close neighbours.

South Korean President Lee Myung-Bak, on an official state visit to China, was due to tour Sichuan on Friday, making him the first foreign head of state to visit the quake zone.

But old sensitivities remained. Japan said Friday it had decided against sending military aircraft to China to deliver aid for victims of this month's devastating earthquake.

Japan plans to charter a commercial flight instead to fly tents and other relief supplies to China.

The death toll from the quake has reached 68,558, with another 18,618 missing, the government said on Friday.