China's Defence Ministry said 71 more military choppers and 30 civil helicopters are being sent to the disaster zone while the army is planning large-scale air-dropping of relief material, official Xinhua News Agency reported.

Premier Wen Jiabao ordered 30,000 additional troops to join the rescue operation.

Soldiers were also rushed to shore up a two-year-old dam in the worst hit Sichuan province, which developed several cracks in the quake, threatening to wash away people downstream.

Officials later said the dam was safe, reports said.

The Ministry of Water Resources issued a notice to check reservoirs nationwide, while the economic planning agency said nearly 400 dams, most of them small, was damaged.

In Beichuan county, a swelling river, blocked by a landslide, was threatening to swamp rescue teams. The teams along with residents were evacuated to higher ground amid fears that the river could burst.

The state media reported that other than the confirmed toll of nearly 15,000, about 26,000 people remained trapped under collapsed buildings from Monday's magnitude 7.8 earthquake. Some 1,400 more are missing.

Forty-four counties and districts in Sichuan were severely affected by the quake. About half of the 20 million people living in these areas were directly affected by the quake, Xinhua said.

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

"Life is the most precious thing, we must be amenable to the people and to history."

The government made a public appeal for donation of thousands of shovels, hammers and other tools, saying some rescuers were sifting through debris with their bare hands.

The industry ministry issued a list of 31 types of urgently needed equipment. It needed up to 50,000 units of some of the equipment listed in the appeal, the ministry said.

China also said it will allow emergency rescue teams from Japan to aid quake relief efforts, the first country from which it has accepted such help.