BEIJING — Atlantans living in China's western Sichuan province described chaotic scenes after a massive earthquake rocked the region, collapsing schools, homes and roads and killing at least 12,000 people.

The 7.9 magnitude quake struck a poor mountainous area of Sichuan on Monday afternoon and sent tremors radiating hundreds of miles into neighboring nations and coastal cities.

In Chengdu, Sichuan's capital, 323 people were killed and several buildings tumbled, state media reported.

"It was pretty crazy," said Ben Charlton, a 37-year-old researcher working with Zoo Atlanta and a giant panda research base in Chengdu.

"Everything started shaking and people were running outside in their underwear and standing in the middle of the road waiting for it to subside."

Maryanne Heard, a 23-year-old from North Druid Hills who was in Chengdu to study reproductive behavior among pandas for Zoo Atlanta, said that while she had seen little structural damage in Chengdu, a city of 10 million people, roads leading west had been damaged, blocking rescue efforts and raising fears that the death toll could rise.

Across the region, communities dealt with their private tragedies.

In Mianzhu, a small city at the edge of mountains west of Chengdu, 10,000 people were buried in collapsed buildings, the Xinhua News Agency reported.

Rescue workers reached Wenchuan, a county of 106,000 people at the earthquake's epicenter, late on Tuesday and found that many structures were destroyed, Xinhua reported.

In another county, 1,000 students and teachers were trapped in the remains of their six-story school building. In Gansu, a province north of Sichuan, a 40-car freight train including 13 gasoline tankers derailed and exploded.

Sarah Bexell, a former Zoo Atlanta conservation expert who now works for the Chengdu Research Base of Giant Panda Breeding, was traveling in southern China when the earthquake struck.

Because roads and telephone lines had been cut, "no one can get through to Wolong," a beautiful town nestled within one of China?s largest giant panda reserves, she said.

The Associated Press reported that more than two dozen British and American tourists who were thought to be panda-watching in the area around Wolong are missing.

The epicenter "was right in the middle of all the panda reserves that we work with and we can't contact anyone,? said Bexell, who lived in Buckhead and near Grant Park until she moved to China in 2006.

"It's very worrying. We have no idea about what's happened to our colleagues and even the pandas."

One hundred and eleven U.S. Peace Corps volunteers currently working in Sichuan and in neighboring provinces and municipalities in western China were all safe, said Shirley Hinrichs, the organization's Chengdu-based program officer.

"Everyone is accounted for," she said. "I believe that when we talked with everyone, they were in their apartments.?

Chinese Premier Wen Jiabao urged rescue personnel to work faster. Thirty-four thousand police and soldiers are working in effected areas and should quickly clear roads to the worst hit regions, he said, according to state media.

"Time is most important thing for saving lives," Wen said. "We should not waste one moment."