At least 300 people have died and thousands are feared hurt after a magnitude-6.9 earthquake struck China's Qinghai province, officials say.
The powerful tremor hit remote Yushu county, 800km (500 miles) south-west of provincial capital Xining, at 0749 (2349 GMT), at a shallow depth of 10km.
Most of the buildings in the worst-hit town of Jiegu town were wrecked, and landslides have cut off roads.
Rescue crews were travelling to Yushu, hundreds of miles from a major airport.
About 5,000 specialist quake rescuers have been dispatched from neighbouring provinces, with the first teams expected to reach Yushu within hours.
A local official in the worst-hit town of Jiegu told the BBC that the loss from the quake was huge, and that almost all of the buildings in the town had been destroyed.
"The death toll will definitely go up," he said.
Another official told China's state news agency Xinhua that the area was in urgent need of help.
"The streets in Jiegu are thronged with panic - injured people, with many bleeding in the head," Zhuo Huaxia told Xinhua.
"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."
Many of the buildings in Yushu, a county with a largely Tibetan population of about 250,000, were thought to be made from wood.
In 2008, a huge quake struck neighbouring Sichuan province which left 87,000 people dead or missing.
Quake-prone region
Karsum Nyima, from Yushu county's TV station, told China's state-run CCTV that school students had been assembled in outside playgrounds, although school buildings had not collapsed.
"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 within 100km of the area since 1976.
The region, which is home to 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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