More than 320 rescuers have joined the search for 22 workers who were trapped underground after an explosion at a gold mine under construction in Yantai, Shandong province, according to news briefings held on Tuesday by the rescue team.

The explosion occurred 240 meters away from the mine entry at 2 pm on Sunday in Xicheng town of Qixia city, which is administered by Yantai city.

The cause of the explosion is under investigation.

The blast trapped 22 workers who were working more than 600 meters away from the entry, said Sun Shufu, head of the emergency management bureau of Yantai.

Due to damage to the communication system caused by the blast, rescuers have so far been unable to contact the trapped workers.

The Ministry of Emergency Management sent an expert team to the site to guide the rescue work on Tuesday morning.

Drilling machines have been installed at the site as part of the rescue effort. A drill pipe reached a depth of around 400 meters by 10 am on Tuesday, and rescuers equipped with respirators are being lowered to clear obstacles, Sun said.

The rescue effort has been slow because poisonous gas in the mine cannot be dispersed quickly, and drills have been blocked by obstacles that will take time to be cleared, he said.

By 4 pm Tuesday, the rescue buckets that take rescuers down have been lowered to 180.5 meters underground, and the system to detect sounds has reached a depth of 370 meters, said Sun.

The rescue team plans to drill three holes from the ground to provide air and supplies to the trapped workers, and one driller is currently working on drilling one of the holes, said Wang Zongwen, a member of the expert panel of the rescue team.

The gold mine is owned by Shandong Wucailong Investment Co.

The local government received an accident report from the mining company 30 hours after the accident occurred, according to Xinhua News Agency on Tuesday.

The company could be held accountable for the late report, Xinhua said.