The rescue teams encountered coal subsidence at the state-run mine, which is located in the northern Black Sea province of Zonguldak.

Turkish Coal Board general manager Burhan Inan reportedly said rescue teams had cleared one obstacle and envisaged it would take four days to break through the further 20m-long subsidence, according to Reuters.

“We are working to shorten that time,” Inan was quoted as telling reporters from the state-run Anatolian news agency.

The 30 miners have been trapped 540m underground since Monday, when a mysterious explosion rocked the complex. Since then, rescue workers say they have had no communication with the miners.

The rescue effort has been complicated by the fact that one of the elevators running down to the lower shafts was damaged by the explosion. Another elevator only reaches to a depth of 510m. About 400 rescue workers are on the scene.

Prime Minister Tayyip Erdogan arrived in Zonguldak on Wednesday afternoon to inspect the rescue efforts, amid growing concerns about the workers' chances of survival.

“Naturally as the time period extends it increases further our concerns about our worker brothers down there,” Energy Minister Taner Yildiz reportedly said.

President Abdullah Gul has given the order for the State Audit Board to conduct a comprehensive investigation into recent accidents that have occurred in the mining sector, a statement on his website said.