A powerful earthquake is feared to have killed at least 8700 people in south-west China and trapped countless others.

But with the death toll predicted to rise rapidly, there were fears the number of dead could reach tens of thousands.

The worst earthquake in the area for more than 30 years emptied office buildings in the Chinese capital Beijing, 930 miles to the north, and was also felt as far away as Vietnam.

The official Chinese state news agency Xinhua reported that 8533 people had died in Sichuan province alone and said more than 200 others died in three other provinces and the mega-city of Chongqing after the 7.8-magnitude earthquake struck at 2.28pm local time on Monday.

The news agency said 80% of the buildings were reported to have collapsed to the east, in Beichuan county, where 10,000 people were injured and up to 5000 were dead. It and other state media said a chemical plant in Shifang city was badly affected burying hundreds of people and spilling more than 80 tonnes of toxic liquid ammonia.

Premier Wen Jiabao called the earthquake "a major geological disaster" and flew into the Sichuan capital of Chengdu to oversee rescue and relief operations. Though slow to release information at first, the Chinese government and its state media said about 20,000 soldiers, police and reservists were sent to the disaster area, with some going on foot because roads were impassable.

The earthquake was the deadliest in the area since 1976.

Much of the area has been closed to foreign media and travellers since, compounding the difficulties of getting information from the region. Roads north from Chengdu to the disaster area were sealed off early yesterday to all but emergency convoys.

David Miliband, the Foreign Secretary, said: "Emerging news of the loss of life and devastation in China is deeply saddening. My thoughts are with the Chinese people and those families who have suffered loss. The UK stands ready to assist."

US President George W Bush also offered humanitarian help.