Five of the six survivors of a major traffic accident that killed 15 people in Southwest China's Sichuan province were in critical condition on Tuesday evening.

"They have fractures in their skulls, arms and legs after falling into a 100-meter-deep ravine," said a surgeon named Huang at the Aba Tibetan and Qiang autonomous prefecture hospital in western Sichuan.

Rescuers work at the accident site after a bus plunged into a deep valley at Zhegu Mountain in Aba Tibetan and Qiang autonomous prefecture, Southwest China's Sichuan province, on Tuesday. [Li Qiaoqiao / Xinhua]

"More than 300 doctors and nurses in the hospital had been mobilized to provide first aid for the survivors. And the prefecture police and safety watchdog are investigating the cause of the accident," said Yang Changqing, deputy chief of the prefecture government.

Fifteen people, including the driver Wang Qian, a native of Ma'erkang county in Aba, were killed around 12:30 pm when a bus from Chengdu overturned.

"The bus, carrying 21 people, fell into a ravine after passing a tunnel in Zhegu Mountain in Ma'erkang. The road conditions are very good. I don't know how the bus overturned," said Yuan Jian, a witness.

According to Yang, nearly 200 police officers, firefighters and doctors rushed to the scene with 10 ambulances from Ma'erkang.

"By 3:30 pm, all of those injured had been sent to the Aba Tibetan and Qiang autonomous prefecture People's Hospital," said Yuan, who added the bus was not overloaded.

Zhegu Mountain tunnel, 3,000 meters above sea level, is the longest highland road tunnel in the country. Completed in 2004, it is about 50 km from Ma'erkang.

Contact the writer at huangzhiling@chinadaily.com.cn.