China's state-run news agency Xinhua reported local quake-relief officials had put the death toll at 589 as of 1:20 a.m. local time Thursday.
Along with those killed and injured in the quake that struck Qinghai province's Yushu County at 7:49 a.m. Wednesday, an undetermined number, including students, remained trapped in the rubble of homes and other structures in the sparsely populated region, senior county official Huang Limin told Xinhua.
Authorities said there had been at least 18 aftershocks, some in excess of magnitude-6.
"Many students are buried under the debris due to building collapse at a vocational school," one official said. One teacher at a boarding school said at least five students had died.
"We have to mainly rely on our hands to clear away the debris as we have no large excavating machines," a police official said. "We have no medical equipment either."
Authorities said about 85 percent of the homes in the region also were destroyed by the quake, whose magnitude was measured at 7.1 by the China Earthquake Networks Center and at 6.9 by the U.S. Geological Survey.
The epicenter was at a depth of about 20.5 miles west of Jiegu township, the county seat and the province's ethnic Tibetan region. Jiegu is about 500 miles from Qinghai's capital Xining. The USGS said the epicenter was about 150 miles northwest of Qamdo in nearby Tibet.
Officials said "many others" were still trapped under the rubble as 5,000 rescuers, including 700 soldiers, worked to save them.
"Our first task is to save students. Schools are always places that have many people," Xinhua quoted army officer Kang Zifu, a member of the rescue team, as saying.
"I can see injured people everywhere. The biggest problem now is that we lack tents, we lack medical equipment, medicine and medical workers," one county official said.
Yushu County, with an area of about 103,000 square miles and an average elevation of 13,123 feet, has a population of about 252,700 people, 97 percent of them Tibetans, Xinhua said.
"The houses here are almost all made of wood and earthen walls, which will easily collapse when a quake happens," Karsum Nyima, news director at a television station in Yushu, told Xinhua.
The quake also was felt in the neighboring provinces with some damage to structures reported.
China Eastern Airlines released two of its Airbuses to help transport rescuers and relief supplies.
"Aftershocks above 6 magnitude are still likely to happen in the coming several days," warned Liu Jie of the China Earthquake Networks Center.
The region was jolted by a 4.7-magnitude quake 2 hours earlier at a depth of about 3.7 miles. It was not known if there were any injuries from that tremor.
In one of the bigger earthquake disasters in China, the country's southern region was devastated by an 8-magnitude temblor May 12, 2008, in which about 70,000 died and 18,000 were listed as missing, Xinhua said.