A military helicopter crashed while evacuating injured survivors of China's earthquake, official media said Sunday, underlining the lingering risks of a disaster that has left more than 68,000 dead.

The chopper crashed Saturday amid fog and strong turbulence as its crew of four was evacuating 10 injured residents from quake-hit areas in southwestern China's Sichuan province, state-run Xinhua news agency said.

No information on casualties was immediately available.

The crash highlights the ongoing challenges as China seeks to respond to a tragedy that has also left nearly 15 million people homeless and raised fears of epidemic outbreaks in affected areas.

The helicopter had ferried a team of military experts on disease outbreaks to Li county and was returning with the injured residents when it lost contact with ground command, Xinhua reported.

It later confirmed the chooper had crashed.

President Hu Jintao, touring quake-stricken areas in neighbouring Shaanxi province, immediately ordered a search and rescue operation, Xinhua said.

The overall death toll from the earthquake, China's worst natural disaster in a generation, reached 68,977 on Saturday, with another 17,974 missing, the government said.

Meanwhile, hundreds of thousands of Chinese waited anxiously for drainage work on a menacing lake created by the May 12 quake to take effect.

Army and police crews who had toiled for a week to dig a diversion channel for the Tangjiashan "quake lake" wrapped up late Saturday and authorities said the rising waters could begin spilling into the channel as early as Sunday.

The lake, formed when a huge landslide blocked the Jian River, has emerged as the most serious lingering threat to the region's traumatised citizens, as it poses a flood risk to areas populated by more than one million people.

More than 197,000 people had been evacuated in case of flooding, officials told AFP, but a total of 1.3 million stood ready to move should the controlled release of water turn into a flood.

Authorities had originally planned to blast the blockage away with dynamite but feared that could cause all the rubble to give away, thus letting loose a deluge.

They now hope to slowly drain away water in the lake, whose level has been rising by nearly two metres a day.

"About 1.3 million people are prepared for evacuation," a disaster relief official in Mianyang city downstream from the lake, who gave only his surname Pu, told AFP on Saturday.

"If the water drainage goes as planned peacefully, they won't have to."

State television broadcast images Saturday of hundreds of troops and heavy equipment being helicoptered away from the lake, their work complete. Hundreds more were to be flown out Sunday, it added.

An AFP journalist in Beichuan county downstream from Tangjiashan saw about 15 buses carrying armed police driving away from the zone Sunday morning.

Officials have expressed confidence that the risk of a disastrous flood is now remote.

"The lake problem is under control, we do not have any fears there will be an uncontrollable flood," said Han Guijun, a top official in quake-devastated Beichuan county.

There were also 33 other lakes in Sichuan created by the quake, 28 of which were at risk of bursting, Xinhua reported earlier.