An additional team of 43 people and more machinery were added to the rescue effort on Thursday for 22 gold miners trapped underground after an explosion at a mine in Yantai, Shandong province.

The blast occurred at 2 pm on Sunday at the Hushan mine in Qixia, It trapped the workers more than 600 meters from the mine opening, rescuers said.

By Thursday noon, more than 390 rescuers and more than 70 machines had joined the rescue work.

A major difficulty is clearing large obstacles heaped in the shaft, which was used for ventilation, according to information provided by the rescue command base.

The shaft is so narrow that no more than three people at a time can work on clearing the debris.

In addition to those trying to open the damaged shaft, machines are at work around the clock elsewhere, urgently drilling holes in search of an alternative route to the trapped miners.

One of the holes had reached a depth of 420 meters as of noon on Thursday.

Rescuers are keeping an eye on water levels as they drill. Water in the wrong location could subject the trapped workers to new dangers.