A series of earthquakes has collapsed houses and triggered landslides in a remote mountainous part of southwestern China, killing at least 50 people with the toll expected to rise.

Damage was preventing rescuers from reaching some outlying areas, and communications were disrupted.

The quakes started with a 5.6-magnitude shock before 11:30 am along the borders of Guizhou and Yunnan provinces, and another equally big quake struck shortly after noon followed by more than 60 aftershocks, Chinese and US government seismologists said. Though of moderate strength, the quakes were shallow, which often causes more damage.

Hardest hit was Yiliang County, where 49 of the 50 deaths occurred, said Yunnan province government agencies and state media. Another 150 people in the county were injured, said Zhang Junwei, a spokesman for the provincial seismology bureau.

China Central Television showed roads littered with rocks and boulders, and pillars of dust rising over hillcrests - signs of landslides.

Footage showed a couple hundred people crowding into what looked like a school athletic field in Yiliang's county seat, a sizeable city spread along a river in a valley bottom.

With some roads impassable, rescuers had yet to reach some outlying villages and towns, the official Xinhua News Agency said.

Though quakes in the area occur frequently, buildings in rural areas and China's fast-growing smaller cities and towns are often constructed poorly.

In 2008, a magnitude-7.9 quake that hit Sichuan province, just north of Yunnan, killed nearly 90,000 people, with many of the deaths blamed on poorly built structures, including schools.

Xinhua said Friday's quakes destroyed or damaged 20,000 homes. The Yunnan seismology bureau said and more than 100,000 people were evacuated from their homes. All told, Xinhua said, 700,000 people had their lives disrupted by the quake.

In Luozehe, a town in Yiliang near a zinc mine, residents and state media said boulders hurtled off hillsides and houses collapsed.

"It is scary. My brother was killed by falling rocks. The aftershocks struck again and again. We are so afraid," Xinhua quoted miner Peng Zhuwen as saying.

A government official in Jiaokui town said a large number of houses had collapsed.

"The casualty number is still being compiled. I don't know what was like for the other towns, but my town got hit badly," he said. Like many Chinese officials he refused to give his name.

Mobile phone services were down and regular phone lines disrupted. Phones were cut off to clinics in four villages in Qiaoshan, another town in Yiliang, which has about half a million people.

Xinhua said thousands of tents, blankets and coats were being shipped to the area.

It said that so far no casualties had been reported in neighboring Guizhou, but that homes had been damaged or destroyed there.

Friday's quakes were relatively shallow, about 10 kilometers deep, creating an intense shaking even at a lower magnitude.

By comparison, the 7.6-magnitude quake that struck Costa Rica this week was 41 kilometers below the surface, and combined with strict building codes, that kept damage and deaths to a minimum.

- AP