XI'AN - Toxic gas inside a copper mine has left five workers dead in northwest China's Shaanxi Province, local authorities confirmed on Wednesday.

The accident occurred around 3:40 p.m. on Tuesday when the workers, all from Wenzhou No. 2 Pit Engineering Company, were working at Dadi Mining Company in Lueyang County, according to a statement from the county government.

An initial investigation showed that a worker surnamed Feng entered a shaft that has been sealed for half a year to fetch clothes that he had left there before the shaft was sealed.

Three of Feng's co-workers, surnamed Kang, Zhou and Qin, entered the shaft to look for him after they noticed that he had not come out.

The three workers attempted to retreat after they realized the abnormal situation inside the pit due to the high density of toxic gas.

However, only Kang succeeded in getting out and the other two were felled by toxic gas.

Another two workers, surnamed Zhang and Chen, then descended into the shaft to rescue the others but were also trapped inside.

By 7:20 p.m., all five workers had been pulled out of the mine and sent to local hospitals. All of them died later after rescue efforts failed.

Production at the mine has been suspended for safety checks and overhaul, according to the statement.

Further investigation into the accident is underway.