Chinese TV has been showing the damage to buildings
At least 300 people have died and others are trapped under rubble after a magnitude-6.9 earthquake struck China's Qinghai province, officials say.
The powerful tremor hit remote Yushu county, 500km (310 miles) south-west of provincial capital Xining, at 0749 (2349 GMT), at a shallow depth of 10km.
Chinese TV showed wrecked buildings and people scrabbling through debris.
Officials ordered rescue crews and supplies to Yushu, but the area is hundreds of miles from an airport.
"Soldiers have been dispatched to save the people buried in the collapsed houses," local official Huang Limin was quoted as saying by China's state news agency Xinhua.
A Yushu resident told Reuters news agency rescuers were trying to pull survivors from buildings.
"A lot of one-storey houses have collapsed. Taller buildings have held up, but there are big cracks in them," he said.
One official told journalists more than 85% of buildings near the epicentre had collapsed.
The official added that "a lot of students" had been buried after part of a vocational school collapsed.
Many of the buildings in Yushu were though to be made from wood.
In 2008, a huge quake struck neighbouring Sichuan province, which left 87,000 people dead or missing.
Karsum Nyima, the Yushu county television station's deputy head of news, told China's state-run CCTV that school students had been assembled in outside playgrounds, although school buildings had not collapsed.
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.
"In a flash, the houses went down. 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."
Zhuo De, an ethnic Tibetan resident of Yushu, who spoke by phone from the capital of Qinghai province, Xining, said there could be many more casualties.
"The homes are built with thick walls and are strong, but if they collapsed they could hurt many people inside," he said.
The remote high-altitude region is prone to earthquakes, but officials from the US Geological Survey said this was the strongest quake the area had seen since 1976.
The region, which is home to ethnic Mongolians and Tibetan farmers and herdsmen, is dotted with coal, tin, lead and copper mines.
After the Sichuan quake, five million people were left homeless, and officials estimated rebuilding work would take at least three years.
The government later punished people who had compiled lists of the victims and had suggested shoddy school-building was partly to blame for the high death toll.
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