The rescue effort faced a major setback when a fresh collapse blocked the ventilation shaft rescue workers were descending. No contact has yet been made with the miners, trapped 7 km inside the winding mine, and around 300 meters vertically underground.

Authorities hope they managed to reach an underground shelter with oxygen and are rationing food and water.

The hope is now to find them and pass them fresh supplies of water and food and communication lines down a narrow drill hole until experts find a way to extract them from the small gold and copper mine.

"The situation is not easy, I must tell the truth," President Sebastian Pinera said on Sunday after cutting short a visit to Colombia to fly home to meet families of the trapped miners.

Rescuers face a race against time to reach the miners, who, if they reached the shelter, are huddled in a space the size of a small apartment.