By Mark Stone, China correspondent
At least 179 people are believed to have been killed and more than 6,700 injured after an earthquake struck China's province of Sichuan.
Chinese news agency Xinhua said the quake, measured at varying magnitudes between 6.6 and 7.0, struck at 8.02am local time close to the city of Ya'an in Lushan county and the epicentre had a depth of a little less than eight miles (13km).
It started landslides, destroyed buildings, and triggered a major rescue operation involving emergency services and more than 2,000 troops.
The rescue effort is understood to have been hampered by aftershocks and landslides. Some of the tremors which followed the main quake have been almost as strong.
According to local media reports, a vehicle carrying 17 soldiers fell off a cliff and into a river as it travelled to the epicentre. One soldier was killed and seven more injured, three seriously.
But extraordinary stories of survival are already emerging. According to one local media report, a mother managed to lift concrete weighing 50kg to rescue her son from the rubble.
Another report described how a woman gave birth at a bike shelter just a few hours after the quake hit.
According to the People's Daily newspaper, fire-fighters dug out a mother holding her baby in her arms from the rubble. She survived, but her baby sadly died.
Tremors have been felt in neighbouring provinces and in the provincial capital city of Chengdu. Users of Weibo, China's microblogging service, have posted images of damaged buildings and described scenes of panic as residents rushed outside.
A resident in Chengdu said he was on the 13th floor of a building when he felt the quake. The building shook for about 20 seconds and he saw tiles fall from nearby buildings.
Local seismologists registered the quake at magnitude 7.0 while the US Geological Survey (USGS) put it at 6.6.
The USGS said "significant" casualties were likely and "extensive damage is probable and disaster is likely widespread".
"Past events with this alert level have required a national or international-level response," it added.
A Chinese General, flying over one area in a helicopter, told local media that as many as 50 percent of buildings in one area had collapsed.
China's new Premiere Li Keqiang is flying to the region to survey the damage. The quake is the first big test for the new Chinese leadership. Their ability to respond to it is likely to be scrutinised on an ever growing social media forum.
The epicentre is close to the location of the 2008 earthquake which killed 68,000 people. That quake had a greater magnitude of 7.9 but its epicentre was further underground.
The 2008 disaster destroyed large swathes of the province along with areas of neighbouring Shaanxi and Gansu. The government faced significant criticism for allowing badly constructed buildings to be erected and for covering up their failings.
International offers of assistance have already been received.
British Foreign Secretary William Hague sent the condolences of the nation.
Mr Hague said: "Today's events will no doubt bring back terrible memories of the catastrophic earthquake in the same province in 2008.
"My thoughts are also with the Chinese authorities and the emergency services as they continue their rescue efforts. We are in contact with local authorities through our Consulate-General in Chongqing and will continue to follow the situation closely."
The Russian President Vladimir Putin sent China’s President Xi Jinping a telegram expressing his condolences and offering help.
Twin tremors in neighbouring Yunnan province last September triggered landslides that left at least 80 people dead.
A 5.5 magnitude quake in Yunnan last June killed four people and injured more than 100.
Another 5.4 tremor the year before near the border with Burma left 25 people dead and injured 250.
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