The death toll appeared likely to climb in China's worst earthquake for over three decades as troops struggled on foot to reach the worst-hit area of Wenchuan, some 100 km (62 miles) from the Sichuan's provincial capital Chengdu.

Officials said there was no word from three townships nearest epicentre in Wenchuan, a hilly county of 112,000 people.

About 900 teenagers were buried under a three-storey school building in the Sichuan city of Dujiangyan. Premier Wen Jiabao, who rushed there, bowed three times in grief before some of the 50 bodies already pulled out, Xinhua news agency reported.

Xinhua said at another Dujiangyan school 420 students were trapped and workers had so far been able to rescue less than 100.

"Not one minute can be wasted," Wen said. "One minute, one second could mean a child's life."

In Chengdu, many residents slept outside or in cars, fearing more tremors in the city where at least 45 people died and 600 were injured.

The government has rushed troops and medical teams to dig for survivors and treat the injured. Sometimes struggling to contain his emotions, Wen vowed to spare no effort while urging crying and injured residents to stay calm.

Severed roads and rail lines blocked the way to Wenchuan, and local officials described crumpled houses, landslides and scenes of desperation.

"We are in urgent need of tents, food, medicine and satellite communications equipment," the Communist Party chief of Wenchuan, Wang Bin said, according to Xinhua.

TOWNSHIPS COLLAPSE

Most farmers' homes in two townships had collapsed and there was no word from the three townships nearest the epicentre, which have a population of 24,000, the report added. So far Wenchuan has reported 15 dead, a number likely to rise steeply.

More than 7,000 may have died in Sichuan's Beichuan Qiang Autonomous County, where 80 percent of the buildings were destroyed, Sichuan television said. Beichuan has a population of 161,000, meaning about one in 10 there were killed or injured.

"Even if it means walking in, we must enter the worst-hit areas as quickly as possible," Wen said, according to Xinhua.

But a paramilitary officer marching with a hundred troops towards Wenchuan described a devastated landscape that is likely to yield many dead and to frustrate rescuers.

"I have seen many collapsed civilian houses and the rocks dropped from mountains on the roadside are everywhere," said the People's Armed Police officer Liu Zaiyuan, according to Xinhua.

Thunderstorms forecast in Sichuan for Tuesday could make rescue attempts more difficult and dislodge more loose rocks.

Most phone lines in Wenchuan were down and a website for the region's Aba prefecture said the quake had cut several major highways and communications were largely severed in 11 counties.

Landslides had cut off three major rail lines leading to Chengdu, stranding 31 passenger trains and 149 cargo trains, Xinhua said, but no casualties had been reported.

The US Geological Survey said the main quake struck at 0628 GMT at a depth of 10 km (6 miles).

Its force was felt across much of China and caused buildings to sway in Beijing and Shanghai and as far away as the Thai capital Bangkok.

SHOCK TO REGION

The quake was another shock to the region already trying to cope with the devastation of a cyclone this month in Burma.

Some 1.5 million people in Burma are facing hunger and disease after the cyclone ravaged the Irrawaddy delta, leaving an official toll of 31,938 dead and 29,770 missing.

The Sichuan quake was the worst to hit China since the 1976 Tangshan tremor in north-eastern China where up to 300,000 died.

This time the devastation was worst in hilly farming country, where winding roads can be hard travel even in normal times. The area is near the famed Wolong panda reserve.

The disaster 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.

Tong Chongde, a spokesman for the massive Three Gorges Dam Project near Sichuan, said there was no damage to the structure.

In Shefang city in Sichuan, 6,000 residents were evacuated after two chemical plants were levelled, trapping more than a hundred people and spilling corrosive liquids.

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

Chinese officials and scientists said that Beijing was unlikely to see more aftershocks, Xinhua reported.

In Washington, President George W. Bush said the United States was ready to help. The UN Secretary-General Ban Ki-moon as well as Japan, France, Germany and other powers have also sent messages offering condolences and help.

But for now China is struggling to get its own rescuers where they are most needed, and one international aid expert said the death toll was likely to rise.

"Our biggest concern is children who were in schools and orphanages when the earthquake hit," said Wyndham James, the China country director for the Save the Children charity.

"I can imagine the authorities are releasing only conservative [death toll] figures that are likely to grow."

Some 61 people 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.