A powerful quake measuring 7.8 struck Monday close to densely populated areas of southwestern China, toppling buildings and rattling cities across a large swathe of the country and southeast Asia.

Xinhua reported at least four children were killed when the quake toppled two primary schools in the city of Chongqing near the epicentre, with another 100 injured.

The state-run agency reported that buildings had collapsed in neighbouring Yunnan province, and President Hu Jintao urged an "all-out" effort to rescue victims.

Military troops were ordered to help with the disaster relief work and the international airport at Chengdu, closer to the epicentre, was closed.

Xinhua said Premier Wen Jiabao was on his way to the region.

The quake struck 93 kilometres (58 miles) from Chengdu, capital of Sichuan province and a major population centre with more than 12 million people, and about 260 kilometres from Chongqing and its 30 million.

The State Seismological Bureau located its epicentre as in Wenchuan County, which is home to the Wolong Nature Reserve, the nation's leading research and breeding base for endangered giant pandas.

Buildings shook in Beijing and Shanghai, residents in the cities reported, with many people evacuating tower blocks and rushing onto the street, although there were no immediate reports of damage there.

Tremors were also felt in Bangkok, Hong Kong, Hanoi and Taipei, residents there said.

China's state-run Xinhua news agency reported the quake 7.8 on the Richter scale. The US Geological Survey, which uses a difference scale, also measured it at 7.8 after earlier revisions.

Xinhua said the quake made buildings collapse in the neighbouring province of Yunnan, adding there were no reports yet of casualties.

A reporter for CCTV news in Chengdu said residents had poured out onto the streets but public transport and electricity supplies remained operational.

However, the tremor appeared to have disrupted cellular telecommunications in the city, he added.

State television also said there appeared to be no infrastructure problems in Chongqing.

The phone network in Chengdu and elsewhere around the country appeared to suffer a meltdown as people throughout China tried to find out what happened, making it extremely difficult to contact residents.

The quake struck shortly before 2.30 pm (0630 GMT), according to the USGS, at a depth of just 10 kilometres (six miles).

Two residents near downtown Chengdu that AFP contacted by phone said they felt a violent shaking that threw glassware to the floor and toppled street lights.

However, they were not sure whether the quake had caused significant damage in the city as they had not yet ventured far from their own home.

Further information could not be obtained as the connection was cut.

The quake was felt in the Taiwanese capital Taipei, where buildings swayed for half a minute, and in the southern Chinese territory of Hong Kong.

In Hanoi, residents said some high buildings shook for around five minutes but there were no reports of damage.

The last powerful quake to hit China was on March 21, a 7.2 magnitude quake which struck near the northwestern city of Hotan in Xinjiang province.

A reporter for Hong Kong-based Cable News based in Kunming, the capital of southwestern Yunnan province which borders Sichuan, said he saw many people running onto the streets to find out what had happened.

"One office worker working on the 11th floor of a building in Kunming told me that the lamps in his office were shaking and he felt dizzy," the reporter told the local broadcaster.

He added that people in Kunming had tried to call their friends in Sichuan, but the lines were jammed.

The USGS estimate of the depth makes it a so-called "shallow" earthquake -- a category generally known to cause greater damage than deeper ones.

One of the biggest quakes ever recorded was in China in 1976, which killed 242,000 people.

That quake, centred in the northern city of Tangshan, lasted for 15 seconds and flattened 90 percent of buildings. The death toll, out of a population of one million, made it one of the deadliest in the world in the 20th century.

In 1920 and again in 1927, separate quakes in northwest China each left some 200,000 dead.