KUNMING -?A poisonous gas leak killed six workers at a tunnel construction site in southwest China's Yunnan Province Wednesday morning, local authorities confirmed.

The accident happened in a tunnel along a pivotal railway line that will link the cities of Lincang and Dali, according to sources with China Railway 10th Bureau Group Co., Ltd, the contractor of the project.

The sulfureted hydrogen leak was accompanied by an outpouring of water and sludge inside the tunnel, company sources and witnesses said.

The cause of the accident is under investigation.