Key Points A 6.5-magnitude earthquake rattled southwest China's Yunnan province at 4:30 p.m. Sunday. The epicenter is in Longtoushan Township, 23 km southwest of the county seat of Ludian, in Zhaotong city. More than 12,000 houses have been toppled and 30,000 damaged in the quake zone. At least 398 people dead and 1,801 injured Many of the homes that collapsed in Ludian, which has a population of about 429,000, were old and made of brick.

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

About 12,000 homes collapsed when the quake struck Sunday afternoon in impoverished Ludian county, around 370 kilometers (230 miles) northeast of Yunnan's capital, Kunming, 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 center 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 magnitude-6.1 quake struck at 4:30 p.m. on Sunday at a depth of 10 kilometers (6 miles), according to the U.S. Geological Survey. China's earthquake monitoring agency put the magnitude at 6.5.

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.

Repeated aftershocks also were making the rescue work dangerous. The USGS showed four aftershocks of magnitude 4.5 and higher hitting after the initial quake.

The Yunnan Civil Affairs Bureau said on its website that 398 people were killed and 1,801 injured. Another 29,400 people were evacuated, 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. The Red Cross Society of China allocated quilts, jackets and tents for those made homeless by the quake, while Red Cross branches in Hong Kong, Macao and neighboring Sichuan province also sent relief supplies.

Chinese Premier Li Keqiang reached the worst-hit area Monday afternoon to oversee quake relief.

U.N. 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 U.N. was ready to "lend its assistance to efforts to respond to humanitarian needs " and "to mobilize any international support needed."

The White House also offered its condolences.

"Our thoughts and prayers are with the families of those that lost their lives," said National Security Council deputy spokeswoman Bernadette Meehan. "The United States stands ready to assist."

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.