A collapsed expressway tunnel under construction in Yunnan province killed six people, and another six remained trapped as of Wednesday afternoon, local authorities said.

The collapse occurred in the Anshi Tunnel in Lincang city, when water and mud burst through twice at the tunnel exit, first at around 5:30 pm and again at around 6:10 pm on Tuesday, trapping 13 workers inside.

Seven people were rescued, but six of them died. The other worker was rushed to hospital and is in stable condition, the local government said. The rescue is ongoing.

The tunnel is part of the Yunxian-Fengqing expressway, which is under construction by Guizhou Highway Engineering Group Co.

Local government and emergency rescue teams have established an on-site rescue headquarters with seven working groups that are trying to rescue the trapped people and prevent secondary disasters.

The unstable geological conditions and serious weathering of the landscape are making the rescue operation more difficult, the local government said.

Additionally, the vegetation coverage is high and the soil contains a large amount of water, exacerbating the risk of another disaster.

Rescue teams are using life detection equipment and sniffer dogs to search for those who are trapped. Medical teams are on standby.

An investigation is underway to determine the cause of the collapse.

The Lincang government will carry out a citywide safety inspection of all construction projects soon. Projects with safety problems will be ordered to rectify them immediately.