Thousands of people have been killed by Monday's powerful earthquake in just one affected region of central China, its government said, with the toll expected to keep rising as bodies are retrieved from schools, homes and factories.

The death toll quickly rose throughout the day. State-run news agency Xinhua said it had reached 8,533 in Sichuan Province by Monday night, and another 10,000 were believed to be injured.

It reported that authorities were yet to reach Wenchuan County -- which sits at the epicenter of the 7.9-magnitude earthquake with a population of about 112,000 -- because of damage to roads.

In Beichuan County, close to Wenchuan, the number of deaths was estimated at more than 3,000, with 80 percent of the buildings destroyed.

In addition, at least 48 people were killed in the northwest Gansu Province, Xinhua said.

Several hundred students were also feared to be buried in collapsed school buildings, the agency said.

China's Seismological Bureau said the earthquake had affected more than half the country's provinces and municipalities.

U.S. President George W. Bush released a statement saying his country "stands ready to help in any way possible."

"I am particularly saddened by the number of students and children affected by this tragedy," Bush said.

In Sichuan's Shifang city, the quake buried hundreds of people in two collapsed chemical plants, and more than 80 tons of ammonia leaked out, Xinhua said.

The local government evacuated 6,000 civilians from the area after homes and factories were also destroyed.

The worst earthquake to hit China in more 30 years has claimed the lives of nearly 9,000 people. Schools, factories, houses and a hospital in the southwestern province of Sichuan have been destroyed and the number of victims is expected to climb still further.

Close to the epicentre, around 80 per cent of the buildings are in ruins.

The quake, which measured 7.8, struck around 100 kilometres north of the provincial capital, Chengdu, but tremors were felt as far away as Thailand and Vietnam.

Describing the quake as a "major disaster", Premier Wen Jiabao has flown to the area and the president has ordered an "all-out" rescue effort.

The tremor destroyed at least eight schools, burying hundreds of children. About 900 teenagers were trapped in the rubble of one three-storey building.

With thousands of homes destroyed or unsafe, many are spending the night on the streets.

But nightfall, severed communications and blocked roads have hampered rescue efforts in the worst hit areas and the full picture is not yet apparent.