China has ramped up its rescue effort in the earthquake-hit southwest, where more than 40,000 people lay dead or buried under rubble and time was running out to save the living.

As rescue teams worked round the clock, a new threat emerged from creaking dams and reservoirs in the disaster zone which could swamp already flattened towns downstream.

Video: Death toll rises rapidly

Video: Sights and sounds of the quake

All the latest news on the quake

*40,000 dead or missing

*New threat from creaking dams

*30,000 more troops mobilised

*Entire towns flattened

*Chinese planes to drop food

Premier Wen Jiabao ordered another 30,000 troops and 90 helicopters to the area to reinforce search-and-rescue operations, while the military was planning the first large-scale air drops of key supplies.

The full scale of the devastation is only now beginning to emerge as teams hike into the remote epicentre of Monday's powerful 7.9-magnitude earthquake, which levelled whole towns causing untold loss of life.

The national quake relief headquarters said more than 10 million people had been directly affected, the state-run Xinhua news agency reported.

In Yingxiu, a town of about 10,000 people at the epicentre, most buildings had collapsed and a frantic search was under way to find any survivors, an AFP reporter who reached the stricken town said.

Xinhua said large-scale air drops would begin later Thursday, bringing more than 50,000 food packets and tens of thousands of articles of clothing and blankets.

"The worst has been watching people die, knowing you could not do anything for them," said Li Rui, a doctor in the town of Wudu, where beds are laid out in a makeshift hospital at what used to be an open-air market.

The military is also deploying 30 more transport aircraft to ferry rescuers and supplies across Sichuan province, and an AFP reporter saw dozens of army trucks heading to the disaster zone from the provincial capital Chengdu.
Helicopters are essential to the effort as much of the quake area consists of mountain villages and towns cut off by landslides.

"We must use all our forces, and save lives at whatever cost," Wen told a meeting at quake relief headquarters in Dujiangyan.

The destruction around the epicentre in remote Wenchuan county is massive, with whole mountainsides sheared off, highways ripped apart and building after building razed.

Water resources minister Chen Lei warned of "prominent problems in safety and flood prevention" at reservoirs, dammed lakes, hydro-electric plants and other facilities.

Xinhua said spillways had been opened at the Zipingpu Dam near the quake's epicentre to release water following reports of "dangerous cracks" threatening Dujiangyan downstream.

Authorities have so far confirmed nearly 15,000 deaths in Sichuan but that number is expected to soar because of the large numbers trapped under broken homes, schools and factories.

Sichuan's vice governor said nearly 26,000 people were buried, while Xinhua late Wednesday reported 30,000 missing or out of reach in the city of Shifang alone. At least 7,700 people were feared dead in Yingxiu alone.

Rescue teams and volunteers clawed through twisted metal and concrete, dragging out bodies and bloodied survivors of China's worst quake for a generation.

Amidst the tragedy, some tales of hope emerged.

A girl murmuring "save me, save me" was pulled from the concrete heap that was once her school some 50 hours after it collapsed in the quake.

"If anything had happened to her, the voice would haunt me for the rest of my life," a rescue worker told state media.

However at least 270 children were killed at the school when the quake hit as they napped, and a desperate effort was under way to save hundreds more.

China has rebuffed most foreign offers of rescue teams, arguing conditions are "not yet ripe," but said Thursday it would accept a Japanese team bringing sniffer dogs.

They could arrive in China within a day, Japan's foreign ministry said.

"Most people are saved in the first three or four days," Willie McMartin, director of the British-based charity International Rescue Corps, told AFP in Hong Kong where his team is trying to get permission to enter China.

"People can survive up to 15 days, but that is when you are talking about miracles, and miracles do not happen very often."

China launched a mass public appeal for thousands of shovels, hammers and cranes, saying some rescuers were having to shift huge concrete slabs by hand to get to survivors.