KUNMING, Sept. 15 -- Six workers were trapped in debris after a railway tunnel under construction collapsed in southwest China's Yunnan Province on Monday morning, local authorities said.

The accident happened at about 9:30 a.m. in Chuxiong City after one of the support arches for the Guangtong-Dali railway renovation project collapsed, according to the China Railway Eighth Group Co. Ltd. that is overseeing the construction project.

Rescue is under way. The cause of the accident is being investigated.