CHONGQING - Six workers suffocated to death in a smoking railway tunnel under construction in southwestern Chongqing municipality early Friday morning, local authorities said.

The accident happened at 12:30 am after welders inadvertently ignited rubber materials, unleashing toxic fumes into the tunnel in Chongqing's Yongchuan district, according to the local work safety administration.

Two workers died at the scene, and another four perished in the hospital, the administration said.

The tunnel is part of the Chengdu-Chongqing Railway, a key transportation project between the two metropolises of Chengdu and Chongqing.