The BBC's Damian Grammaticas: "A pretty grim picture is being painted"
Some 400 people have died and thousands are feared injured after a magnitude-6.9 quake hit western China's Qinghai province, officials say.
The powerful tremor struck remote Yushu county, 800km (500 miles) south-west of the provincial capital Xining, at 0749 (2349 GMT), at a shallow depth of 10km.
Most of the buildings in the worst-hit town of Jiegu were wrecked, and landslides have cut off roads.
Police said hundreds of survivors had already been pulled from the rubble.
And at least one aid flight had been able to land at the local airport, according to officials.
Senior Yushu official Huang Liming announced the latest death toll of 400, as the extent of the damage became clearer.
Michael Bristow, BBC News, Beijing
The earthquake struck just before 0800 local time - when many people were still at home. It was followed by three aftershocks.
Some media reports say most buildings in the town near the epicentre fell down. Grainy pictures on Chinese television showed rescuers pulling at the rubble of one collapsed structure. Soldiers have been sent to the area to help with the rescue.
The earthquake happened in a remote and sparsely populated area on the Tibetan plateau. The area is regularly hit by earthquakes.
State broadcaster CCTV reports that at least 10,000 people are injured.
A local official in Jiegu told the BBC that almost all of the buildings in the town had been destroyed.
"The death toll will definitely go up," he said.
About 5,000 specialist quake rescuers have been dispatched from neighbouring provinces.
Many people have fled to the surrounding mountains, amid fears that a nearby dam could burst.
State media reported that officials were trying to drain a reservoir after a crack appeared in the dam.
A spokesman for the local government, Zhuo Huaxia, told China's state news agency Xinhua: "The streets in Jiegu are thronged with panic - injured people, with many bleeding in the head.
"Many students are buried under the debris due to building collapse at a vocational school.
"I can see injured people everywhere. The biggest problem now is that we lack tents, we lack medical equipment, medicine and medical workers."
In 2008, a huge quake struck in neighbouring Sichuan province, about 800km from Yushu, which left 87,000 people dead or missing and five million homeless.
The dead included many schoolchildren, prompting a storm of controversy over alleged shoddy construction of school buildings.
Earlier, Karsum Nyima, the local TV station's deputy head of news, told CCTV that houses had gone down "in a flash".
RECENT DEADLY QUAKES
Feb 2010: Magnitude 8.8 quake in central Chile kills at least 450
Jan 2010: About 230,000 die in magnitude 7.0 tremor in Haiti
April 2009: Quake measuring 6.3 in L'Aquila, Italy, kills 300 people
May 2008: 87,000 die in 7.8 scale tremor in Sichuan province, China
Oct 2005: Quake measuring 7.6 hits north Pakistan, killing 73,000
"It was a terrible earthquake. In a small park, there is a Buddhist tower and the top of the tower fell off," he said.
"Everybody is out on the streets, standing in front of their houses, trying to find their family members."
Earthquake survivors are struggling to stay warm in temperatures of about 6C (43F).
Power and water have been cut off, and the road to the local airport is reported to have been blocked by landslides.
After the Sichuan quake, the disaster response was widely praised, but the BBC's Damian Grammaticas in Beijing says the remoteness of Yushu means this rescue effort will pose very different challenges.
Residents of Yushu fled their homes after the quake
Although the high-altitude region is prone to earthquakes, officials from the US Geological Survey said this was the strongest tremor within 100km of the area since 1976.
The Yushu region, home to 250,000 mostly ethnic Tibetans, is dotted with coal, tin, lead and copper mines.
The region is roughly half-way between Xining and Lhasa, about 400km from the Qinghai-Tibet railway line.
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