Four workers have been pulled alive from a tunnel that collapsed at a mine in remote eastern Indonesia Tuesday but 33 others remained trapped, the US operator said. The accident happened at Freeport-McMoRan\'s Grasberg, one of the world\'s biggest gold and copper mines which has been hit by a string of problems including a major 2011 strike that affected production. \"Rescuers have evacuated four of the 37 people believed trapped when part of a tunnel collapsed at an underground training facility,\" the Indonesian subsidiary of Freeport said in a statement. Local police chief Jermias Rontini told AFP those evacuated had been taken to hospital but did not know their conditions. Rontini was not hopeful for those still trapped. \"In the past, similar tunnel collapses caused fatalities as people who were trapped couldn\'t get enough oxygen.\" Freeport said 40 employees and contract workers had been in a classroom at the training facility when the tunnel collapsed at about 7:30 am but three managed to escape immediately. The tunnel is 500 metres away from one of the mining areas, high in the mountains of rugged Papua province. Freeport said rescue operations were \"difficult and will take some time to complete\" but added \"it would spare no effort\" to save the trapped workers. The company added it did not expect production to be affected. The company has not identified the nationalities of those involved in the accident, although the vast majority of the more than 24,000 workers at the mine are Indonesian. Neither police nor Freeport said why the accident happened. The 2011 strike lasted three months and crippled production, only ending once the firm agreed to a huge pay rise. The industrial action sparked a wave of deadly clashes between police and gunmen around the mine, with at least 11 people, all Indonesians, killed. Earlier this month, some 1,100 workers employed by Freeport contractors staged a three-day strike over pay but it caused only minimal disruption to production.