Rescuers dug through shattered homes Monday looking for survivors of a strong earthquake in southern China's Yunnan province as the death toll rose to at least 398 people with more than 1,800 injured.

About 12,000 mostly brick homes collapsed when the quake struck Sunday afternoon in impoverished Ludian county, about 370 kilometres northeast of Yunnan's capital, Kunming, China's official Xinhua News Agency reported.

Ma Yaoqi, an 18-year-old volunteer in the quake zone, said by phone that at least half of the buildings had collapsed on the road from the city centre of Zhaotong to the hardest hit town of Longtou. The rest of the buildings were damaged, she said.

"I saw dead bodies being wrapped in quilts and carried away," said Ma, who had arrived with 20 other volunteers Monday. "Some were wrapped with small quilts. Those must be kids."

Overhead footage of the quake zone shot by state broadcaster CCTV showed older houses flattened but newer multistory buildings still standing.

The U.S. Geological Survey registered the quake at magnitude 6.1 and said it struck at 4:30 p.m. on Sunday at a depth of 10 kilometres, while China's earthquake monitoring agency put the magnitude at 6.5 and the depth at 12 km. The epicentre was in Ludian county township of Longtoushan. 

Rain and thunderstorms were forecast for the area Monday afternoon, complicating efforts to bring tents, water, food and other relief supplies to survivors. Roads had caved in, and rescuers were forced to travel on foot.

Remote communities still cut off

Chinese Premier Li Keqiang visited Longtoushan on Monday afternoon, and issued a series of directives for rescuers, saying it is paramount to maximize efforts to save lives. 

The directives include deploying more experienced medical personnel to quake-stricken areas, repairing main roads to affected areas as soon as possible, and strengthening epidemic prevention.

China Earthquake Premier Li Keqiang

Chinese Premier Li Keqiang comforts a child in the earthquake-stricken town of Longtoushan, in China's Yunnan province, on Monday. (Yao Dawei/Xinhua/Associated Press)

Repeated aftershocks have been making rescue work dangerous. The USGS showed four aftershocks of magnitude 4.5 and higher hitting after the initial quake, while China's agency said a total of 411 aftershocks of all strengths have been recorded.

The Yunnan Civil Affairs Bureau said on its website that 398 people were killed and 1,801 injured. Another 29,400 people were removed from the quake-affected area, CCTV said. The death toll is expected to rise after rescuers reach remote communities to assess casualties.

Many of the homes that collapsed in Ludian, which has a population of about 429,000, were old and made of brick, Xinhua said, adding that electricity and telecommunications were cut off in the county.

The mountainous region where the quake occurred is largely agricultural, with farming and mining the top industries, and is prone to earthquakes.

Relief efforts are underway, with more than 2,500 troops dispatched to the disaster region, Xinhua said. China's earthquake agency upgraded its emergency response to Grade I, the highest level, on Monday.

UN offers support

UN Secretary General Ban Ki-moon offered "his condolences to the Chinese government and the families of those killed," according to a statement from his office. The statement said the UN is ready to "lend its assistance to efforts to respond to humanitarian needs" and "to mobilize any international support needed."

CCTV said the quake was the strongest to hit Yunnan in 14 years.

In 1970, a magnitude-7.7 earthquake in Yunnan killed at least 15,000 people, and a magnitude-7.1 quake in the province killed more than 1,400 in 1974. In September 2012, 81 people died and 821 were injured in a series of quakes in the Yunnan region.

In May 2008, a powerful quake in Sichuan province left nearly 90,000 people dead or missing.

With files from Reuters