Chinese rescuers searched on Monday for a military helicopter carrying injured quake survivors that crashed in heavy fog in the mountains of Sichuan province, state media said.

On board the Mi-171 helicopter were 14 survivors and medical workers and five crew, when it crashed while returning from its 64th relief mission to carry epidemic prevention experts to Lixian county.

It crashed on Saturday near Yingxiu, close to the quake's epicentre, Xinhua news agency said, adding that the helicopter was still missing and no one on board had been found.

China has mobilised its military to unblock roads, clear rubble and deliver food, water and tents to millions displaced by the May 12 earthquake. Rescuers are racing to clear lakes and set up housing before the summer rainy season begins.

As of June 1, officials said the earthquake had killed more than 69,000 people, with nearly 19,000 missing and around 368,500 injured, Xinhua said.

The earthquake has gripped China, unleashing a flood of donations and volunteers to help in relief work.

Troops withdrew from a dangerous "quake lake" formed by a massive landslide at Tangjiashan after clearing a channel for water to flow out from behind the blockage. The water had risen to within seven metres of the lowest point of the unstable natural dam by this weekend, threatening downstream communities.

Trucks trundled across Sichuan with materials for pre-fabricated housing, designed to replace stifling hot and leaky tents.

In Yingxiu itself, dynamite was being used to clear rubble and unsafe buildings. Workers vigorously sprayed disinfectant to guard against epidemics as overcast and muggy weather settled in.

"As time goes by, the major killers of inpatients are multiple organ failure and complicated drug-resistant infection, instead of crush syndrome and acute renal failure in early periods after the quake," Xinhua a quoted Ministry of Health spokesman, Mao Qun'an, as saying.

Mao said that deep burial of the dead in Sichuan had been "scientifically handled" and water sources would not be contaminated.

President Hu Jintao travelled to the southeast corner of Gansu Province, where towns along the fault line were also heavily damaged. He visited a Pakistani medical team, one of several foreign teams doing relief work in China.