Worst-hit areas
 
The toll is expected to rise sharply as authorities and rescue teams make contact with the worst-hit areas of Sichuan.
 
Nearly 900 students might have been buried under the rubble, Xinhua news agency reported.
 
In depth

Students buried under rubble

Pictures: Quake leaves devastation

'No early warning signs'

Earthquakes: A quick guide

In Shifeng, two chemical plants were damaged, burying hundreds of people and forcing the evacuation of more than 6,000 others.
 
Rescuers are trying to locate survivors.
 
State media reported casualties in the provinces of Sichuan, Gansu and Yunnan as well as Chongqing, a municipality of 30 million people that neighbours Sichuan.
 
Buildings toppled in at least six counties near Wenchuan, Xinhua said.
 
About 80 per cent of buildings in Beichun county have collapsed, and about 10,000 people are believed to be injured there.
 
In Beijing, China's capital city, and Shanghai, office workers poured into the streets as the tremor hit.
 
Stadium intact
 
There is no visible damage in Beijing, which will hold the summer Olympics in August, Xinhua, the state news agency said.
 
Wen Jiabao, the Chinese premier, described the incident as a "major disaster" and called for calm.
 
He is travelling to the affected area.
 
Hu Jintao, the Chinese president, has ordered an "all-out" rescue effort, Xinhua reports.
 
Thousands of army troops and members of the People's Armed Police, a paramilitary force, carrying medical supplies are heading to the region, state television said.
 
Airports and railways stations have been closed in Chengdu and state media reports that some parts of the city have been cut off from water and electricity.
 

Al Jazeera's Melissa Chan reported seeing big construction vehicles and military vehicles with People's Liberation Army soldiers moving in to help with the rescue effort.

 

But access to the epicentre of the quake was limited because a major road had been blocked by dirt and rubble, she added.

 

Little information

 

Chan said there was very little information coming out of Wenchuan.

 

She that since the earthquake struck during school hours, there were a lot of children unaccounted for.

 

Key earthquake facts

About 1.4 million earthquakes occur every year – 100 of which cause damage on average.

Predicting the outcome of earthquakes is impossible, although mapping their geographical likelihood is feasible.

The "Ring of Fire" – a horseshoe-shaped band of volcanoes and fault lines circling the edges of the Pacific Ocean – is where most of the largest earthquakes happen.

Central China suffered the world's deadliest recorded earthquake, which killed about 830,000 in 1556.

The largest recorded earthquake was in Chile in 1960. It measured 9.5 on the Richter scale.

Quakes generally occur at depths of less than 80km from the Earth's surface.

Evidence of earthquakes date back to 1831BC, in China's Shandong province.

Source: The US Geological Survey

Locals are worried about aftershocks and gas is being rationed, if available at all, in order to conserve it for the rescue effort.

 

Chang said rescue effort for now aims to provide basic assistance to people, but lack of access is hampering these attempts.

 
Xinhua said there was no immediate impact on the Three Gorges Dam project.
 
Experts have said the weight of the reservoir, which is hundreds of kilometres from Chengdu, could increase the risk of tremors.

 

A statement issued by the local government in Aba prefecture, where the epicentre is located, said buildings had cracked and collapsed, and mountain roads are damaged.

 

The area has a population of about 110,000, the state-run Xinhua news agency reported.

 

Earlier, Jessica Sigala of the US Geological Survey based in Colorado said the China earthquake struck at a relatively shallow depth, so aftershocks could be expected.

 

The country's deadliest quake in recent memory hit the eastern city of Tangshan in 1976.

 

The magnitude 7.8 earthquake was followed by an equally powerful aftershock almost a day later; together they killed around 240,000 people.

 

A magnitude 7.9 earthquake, such as the one that struck southwest China on Monday, is considered a major event, capable of causing widespread damage and injuries in populated areas.

Source: Al Jazeera and agencies