The BBC's Damian Grammaticas: "A pretty grim picture is being painted"
Nearly 600 people have died and thousands been 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.
As survivors in the mountainous region passed the night in freezing temperatures, local rescue officials said the death toll had risen to 589, Chinese state media reported.
Michael Bristow, BBC News, Beijing
The main task now is to get warm clothing, blankets and medical supplies to the people who are out in the open in temperatures that will plunge to below freezing in this high-altitude region.
There is an airport near Jiegu where some planes have landed but it is very small.
Other rescuers are flying in to Xining about 800km away and taking the very long road down to Jiegu. It is going to take them about 12 hours, perhaps more.
It is going to be some time before the supplies that are needed - the heavy-lifting equipment, medical supplies, tents and clothing - get to where they are needed.
State broadcaster CCTV earlier reported that at least 10,000 people were 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.
By nightfall on Wednesday, the local airport was operating with emergency power and receiving flights bringing rescue workers with sniffer dogs.
Crews were working on clearing the road to the airport, which was blocked by landslides.
About 5,000 specialist quake rescuers have been dispatched from neighbouring provinces plus tents, clothing and blankets.
China's Vice-Premier Hui Liangyu has gone to Yushu county to oversee the rescue effort.
Many people have fled to the surrounding mountains, amid fears that a nearby dam could burst in the string of aftershocks hitting the area.
State media reported that officials were trying to drain a reservoir after a crack appeared in the dam.
Most of the damage has occurred to houses built of wood and mud, but some larger concrete buildings have been badly damaged as well.
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."
Another local official told CCTV that contact had been made with 40-50 people buried alive under the rubble of a government building.
Karsum Nyima, the deputy head of news for Yushu TV, told CCTV that houses had gone down "in a flash".
He said: "It was a terrible earthquake. In a small park, there is a Buddhist tower and the top of the tower fell off.
"Everybody is out on the streets, standing in front of their houses, trying to find their family members."
One man living in a town near Jiegu told the BBC the damage was extensive.
"As far as I can see, not many buildings have been left standing... Because the houses are flattened, it is very difficult to dig out survivors or the dead."
Survivors are struggling to stay warm in the mountainous region of about 4,000m (13,000 feet) elevation where temperatures drop below 0C (32F) overnight.
Power and water have been cut off and rain is forecast.
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
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.
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.
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.