Hu Jintao, the Chinese president, called for all-out efforts to save as many people as possible, with more than 5,000 rescuers including soldiers sent to the disaster zone.
The government said it would provide more than $29m in aid.
Al Jazeera's Melissa Chan, who has been travelling with an aid convoy from the provincial capital Xining, to Jiegu, where the quake hit hardest, said it was a long and arduous journey.
"It's going to take a very, very long time to get there," she said.
"There's convoy after convoy of trucks. They will be travelling through the night and then at dawn, hopefully, everybody will get a better sense of what's going on.
"One of the concerns when we get there, apart from the freezing temperatures, is the altitude.
"Many of the rescuers are from low-lying provinces and it can take a few days to get acclimatised to the altitude in Yushu."
The quake, measured at magnitude 7.1 by the China Earthquake Administration and at 6.9 by the US Geological Survey (USGS), struck the Tibetan plateau at 7:49am local time on Wednesday.
It was centered in the mountains that divide Qinghai province from the Tibet Autonomous Region, 380km south-southeast of the city of Golmud.
Quakes are commonplace in the region, but usually cause little loss of life due to the spread of population.
But Wednesday's quake flattened earth-and-wood houses, felled sturdier concrete buildings and cracked a dam, threatening more homes.
"There are 10 people in my family and only four of us escaped," Samdrup Gyatso, 17, told China’s Xinhua state television after his two-storey home crumbled.
"One of my relatives died. All the others are buried under the rubble."
Among the casualties were children trapped under the rubble of at least one collapsed school in Jiegu, with Xinhua reporting that at least five pupils had died.
"Some pupils ran out of dorms alive, and those who had not escaped in time were buried," said one teacher.
|Concrete buildings and wooden homes alike were flattened by the quake [Reuters]
A local fire department official was quoted as telling state television that about 20 children were buried.
In Qinghai, the provincial government said that 5,000 tents and 100,000 thick, cotton coats and heavy blankets were being sent to help survivors cope with strong winds and near-freezing temperatures.
Wu Yong, a local military chief, said medical workers also were urgently needed but that roads leading to the airport had been badly damaged by the quake.
He said rescue efforts were hindered by frequent aftershocks and strong winds.
More than 85 per cent of houses had collapsed in Jiegu, while large cracks have appeared on buildings still standing, Xinhua cited Zhuohuaxia, a local official, as saying.
The town has a population of 100,000.
Exiled Tibetan spiritual leader the Dalai Lama, who was born in Qinghai, said in a statement he was praying for the victims.
"It is my hope that all possible assistance and relief work will reach these people," he said.
A huge quake in May 2008 in China's Sichuan province killed thousands of children when their badly-constructed schools fell on them.
Nearly 87,000 people were killed or missing in the 2008 disaster, the worst in China in more than 30 years.