Chongqing, China - China's most devastating earthquake in three decades killed nearly 9,000 people on Monday, with the toll likely to soar as authorities struggle to reach casualties in large areas cut off from relief.

The earthquake that hit China's southwestern province of Sichuan killed 8,533 people, the official Xinhua news agency said on Monday, citing the provincial government.

The epicenter of the 7.8 magnitude quake was in a mountainous region about 100 km from Sichuan's capital Chengdu, a bustling city of 10 million.

"The road started swaying as I was driving. Rocks fell from the mountains, with dust darkening the sky over the valley," a driver for Sichuan's seismological bureau was quoted by Xinhua as saying, as he was driving near the epicenter.

The quake hit in the middle of the school day, toppling eight schools in the region. Chemical plants and at least one hospital were also flattened, trapping many hundreds, state media said.

About 900 teenagers were buried in the rubble of a collapsed three-storey school building in the Sichuan city of Dujiangyan.

Local villagers had already helped dozens of students out of the ruins and five cranes were excavating the site as anxious parents looked on, Xinhua said.

"Some buried teenagers were struggling to break loose from underneath the ruins while others were crying out for help," the agency said.

Nightfall, severed communications and blocked roads have hampered rescue efforts and the death toll was likely to rise significantly.

Many Buildings Flattened

An estimated 3,000 to 5,000 people were killed in Beichuan Qiang Autonomous County alone, state media said.

As many as 10,000 in Beichuan were feared injured and 80 percent of the buildings there had been destroyed, Xinhua said. There had been more than 300 aftershocks, state television said.

Beichuan's population is 161,000, meaning about one in 10 residents were killed or injured. The county is a part of Mianyang city, and about 160 km (100 miles) from the provincial capital, Chengdu.

Hundreds of people were buried in two collapsed chemical plants in Shifang in Sichuan, the online edition of the official Xinhua news agency said.

About 6,000 people were evacuated, Xinhua said, adding that more than 80 tonnes of highly corrosive liquid ammonia had leaked.

Hundreds of people were buried under rubble in Shifang in Sichuan as several schools, factories and dormitories collapsed during the quake, the official Xinhua news agency said.

Hundreds were also buried under rubble in a collapsed hospital in Dujiangyan city in Sichuan.

The quake's epicenter was in nearby Wenchuan, a mountainous county of about 100,000 people, but its force was enough to cause buildings to sway across China and as far away as the Thai capital Bangkok.

The Sichuan plain is one of China's most fertile agricultural areas, but it relies heavily on an irrigation system linked to the 2,000-year-old Dujiangyan flood control works.

Which means the quake could exacerbate inflation, already running at the fastest pace in 12 years.

The quake is also the worst to hit China in 32 years since the 1976 Tangshan earthquake in northeastern China where up to 300,000 died.

It has come at a bad time for China, which holds the Olympic Games in August, and has been struggling to keep a lid on unrest in ethnic Tibetan areas and the heavily Muslim northeastern Xinjiang region.

The U.S. Geological Survey said on its website (http://earthquake.usgs.gov) the main quake struck at 0628 GMT at a depth of 10 km (6 miles).

In Beijing and Shanghai, office workers poured into the streets as the tremor hit. In the capital, there was no visible damage and the showpiece Bird's Nest Olympic stadium was unscathed.

"All - Out" Rescue Effort

Premier Wen Jiabao arrived in Chengdu and President Hu Jintao ordered an "all-out" rescue effort, Xinhua reported.

Thousands of army troops and paramilitary People's Armed Police carrying medical supplies were also headed to the region, state television said. But a landslide had blocked a mountain road leading to Wenchuan, preventing troops from reaching the scene, state radio said.

In Washington, President George W. Bush said the United States was ready to help.

"I extend my condolences to those injured and to the families of the victims of today's earthquake. I am particularly saddened by the number of students and children affected by this tragedy.

"The United States stands ready to help in any way possible," Bush said in a statement.

At least 45 had died in Chengdu, Xinhua said, citing an official with the local seismological bureau. Another 600 people were injured, 58 of them critically, in the sprawling city.

Some 57 have been confirmed killed in northern Shaanxi, 48 in northwestern Gansu, 50 in Chongqing municipality, and one in Yunnan province, Xinhua said, citing the national headquarters of disaster relief.

(Additional reporting by Beijing and Shanghai bureaux; Editing by Nick Macfie and David Fogarty)