Two people were killed and 31 others are believed to be trapped underground after a tunnel caved in at a gold and copper mine in eastern Indonesia.

At least 40 employees and contract workers were thought to be in an underground training facility when the disaster happened at the Grasberg complex, which is owned by US-based Freeport-McMoRan Copper & Gold Inc.

Three workers managed to escape unhurt while four employees were evacuated and brought to the hospital, local police chief Jermias Rontini told AFP.

Two workers were killed, said Freeport Indonesia, which extended condolences to the victims’ families.

Rescue efforts were underway but “will take some time to complete”. Freeport said “it would spare no effort” to save the trapped workers, whose nationalities were not identified. Neither police nor Freeport gave a cause for the accident.

Freeport said the incident would unlikely impact production at the mine, the world’s third-biggest copper mine ranked by production capacity, according to Bloomberg.