Hundreds of troops were dispatched to Qinghai Province’s Yushu county and some aid shipments from private organizations set off from the provincial capital, Xining.

“I see injured people everywhere. The biggest problem now is that we lack tents, we lack medical equipment, medicine and medical workers,” Zhuohuaxia, a local spokesman, told the Xinhua news agency.

More than 10,000 people were injured and thousands left homeless in freezing conditions after a series of quakes and aftershocks caused many of the low, mud-brick buildings in the county to collapse, residents and state media said.

A dam has “cracked,” Xinhua said, and “workers are trying to prevent the outflow of water.” It was not immediately clear how large the dam was or what damage it could cause if it burst.

The main 6.9 quake was centered in the mountains that divide Qinghai Province from the Tibet Autonomous Region.

“People are very scared,” said Pierre Deve, with Snowland Service Group, a local non-government organization, adding that many had already given up hope for those still trapped.

Some bridges and roads around Yushu have cracked or been cut off completely, which could complicate rescue efforts, state television said. The airport is open, but the road connecting it to the county seat has been heavily damaged, it added.

The Tibetan plateau is regularly shaken by earthquakes, though casualties are usually minimal because few people live there.

Yushu is home to some 100,000 people, spread over a vast area, but the quake struck near the relatively highly populated county seat of Jyeku.

Government officials told state media the majority of houses had been badly damaged.

Photos showed larger concrete buildings mostly intact, with rubble around them. At least five people also died in neighboring Gansu Province, Xinhua said.

President Hu Jintao and Premier Wen Jiabao have demanded no effort be spared in rescue attempts, and sent Vice Premier Hui Liangyu to Qinghai to oversee relief work, state television said.

Xinhua reported that the early morning quake had caused some schools and part of a government office building to cave in. Some vocational school students and primary school students were trapped in the rubble, it said, although residents said most students had been able to flee to playgrounds.