"More than 21,000 buildings were severely damaged and more than 1,200 have collapsed," said the official at the earthquake bureau in Gansu province, where the 5.9 and 5.6 magnitude tremors struck.

The official said 296 people had been severely injured, adding that there have been 371 aftershocks.

The US Geological Survey said the initial 5.9 magnitude quake hit at 7:45 am (2345 GMT Sunday), with its epicentre 151 kilometres (94 miles) west of Beidao in Gansu province at a depth of just 9.8 kilometres.

A second 5.6 magnitude tremor hit the same region at 9:12 am and was 10.1 kilometres deep, USGS said.

A resident of Min county told he was at work at a medicine production plant when the tremor struck and he saw tower blocks shake "ferociously".

"I was in the workshop. I felt violent shaking and so I ran to the yard of the plant immediately," said the man, surnamed Ma.

"Our factory is only one floor. When I came to the yard, I saw a 18 storey building, the tallest in our county, shaking ferociously, especially the 18th floor."

An official surnamed He from the Min county Communist Party committee told there were more than 200 injured in seven affected townships in the area.

While Gansu is one of China's more sparsely populated provinces, Dingxi city, one of the worst affected areas, has a population of about 2.7 million.

A report on the 163.com Internet news portal said 500 troops, including 120 specialised rescuers were on their way to the disaster zone.

Pictures broadcast on state television showed rural villages with rubble-strewn streets.

A total of 380 buildings collapsed and thousands were damaged in Zhang county, according to an online post by the Dingxi local government.