Tower blocks from Beijing to Bangkok rocked and swayed today as an earthquake shook a swath of Asia, with a magnitude of 7.8 in south west China.

Buildings had collapsed in the town of Wenchuan, but there was no immediate word on casualties. Communications to the town appeared to have been severed, with calls failing to connect to both fixed line and mobile telephones.

Premier Wen Jiabao left Beijing and tried to reach the epicentre after the tremor rocked the region at 0628 GMT at a depth of 18 miles. The decision to send such a senior leader hinted that officials may fear a possible high casualty toll from the earthquake near a region with a large Tibetan population that has seen anti-Chinese unrest in recent weeks.

Reports of deaths were beginning to trickle in. Four children died in a suburb of southwestern Chongqing city when the earthquake toppled two primary schools. More than 100 children were injured.

Troops have already been sent to help with disaster relief work in Wenchuan, which has about 111,000 people in the southwestern province of Sichuan. Tian Yixiang, who works for the emergency office of the People’s Liberation Army, said the soldiers would assist the local government there. Wenchuan is best-known as the home of the Wolong Nature Reserve, China's leading research and breeding base for the endangered giant panda. Calls to the base failed to connect.

The government in Aba prefecture of Sichuan province said buildings had collapsed and many were cracked. Mountain roads had been damaged.

The earthquake was felt as far away as Vietnam and Thailand, startling office workers in high-rise buildings. There were no reports of damage. In Beijing, office workers rushed in panic out of towers in the capital’s financial district.

One of China’s tallest buildings, the Jinmao Tower in Shanghai, as well as other skyscrapers were ordered to evacuate after the tremor and aftershocks.

A resident of Mianyang, in Sichuan, said: “The air-conditioning unit fell off the wall. Vases are all broken. The sick in hospital have been moved outside to open fields. There is no electricity and no mobile phone reception. People are afraid of aftershocks."

China’s worst earthquake in modern history was centred in the northern city of Tangshan and killed some 250,000 people on July 27, 1976.

That tremor lasted for 15 seconds and flattened 90 percent of buildings. It was the deadliest in the world of the 20th century.

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