Published: Apr 14, 2010 11:15 AM EDT

BEIJING (AP) - A series of strong earthquakes struck a far western Tibetan area of China on Wednesday, killing at least 400 people and injuring thousands as houses made of mud and wood collapsed, trapping many more, officials said.

Paramilitary police were forced to use shovels to dig through the rubble in a Qinghai province township where most of the homes had been flattened, footage on state television showed.

Officials said excavators were not available and with most of the roads leading to the nearest airport damaged, equipment and rescuers would have a hard time reaching the area.

Downed phone lines, strong winds and frequent aftershocks also hindered rescue efforts, said Wu Yong, a local military chief.

Workers were racing to release water from a reservoir in the disaster area where a crack had formed after the quake to prevent a flood, according to the China Earthquake Administration.

The magnitude-6.9 temblor struck Wednesday morning in an area in the south of Qinghai province, near Tibet, the U.S. Geological Survey said.

The quake was centered on Yushu county, with a population of about 100,000, mostly herders and farmers.

The USGS recorded six temblors in less than three hours, all but one registered 5.0 or higher. days.