Part of a vocational school had also collapsed and "a lot of students are buried underneath", Zhuo added
The quake, which struck at 7:49am (23:49 GMT on Tuesday), was centred in the mountains that divide Qinghai province from the Tibet Autonomous Region, 380km south-southeast of the city of Golmud.
Three aftershocks rattled the area shortly after the quake, with magnitudes of up to 5.8, the USGS reported.
Karsum Nyima, the deputy head of news at Yushu county's television station, said many people were out on the streets, trying to find their family members.
Rescue teams and equipment were being rushed to the area, Xinhua said, but a local military official, Shi Huajie, told state broadcaster CCTV that rescuers were working with limited equipment.
"The difficulty we face is that we don't have any excavators. Many of the people have been buried and our soldiers are trying to pull them out with human labour," Shi said.
"It is very difficult to save people with our bare hands."
Wu Yong, a local military chief, said medical workers were also urgently needed but roads leading from the airport had been badly damaged by the quake, creating difficulties for people and supplies to be flown in.
The Tibetan plateau is regularly shaken by earthquakes, but casualties are usually limited because so few people live there.
The foothills to the south and east of the area are home to herders and Tibetan monasteries, while the area to the north and west is arid and largely empty.