JAKARTA (REUTERS) - Between 35 and 40 percent of workers at Freeport McMoRan Copper & Gold Inc's Indonesian unit returned to work on Saturday to carry out maintenance work after a training tunnel collapse that killed 28 people, a union official said.

The resumption of work was a possible sign that the firm was gearing up towards restarting operations at the world's No 2 copper mine.

Arizona-based Freeport suspended operations at the remote Papua mine on May 15 at a cost estimated at about US$15 million (S$18.7 million) a day in lost production, a day after the tunnel, away from its main operations, fell in on 38 workers.

"Starting today, around 35 to 40 percent of workers have been back to work in Freeport mining facility in Papua, but only for mining facilities and equipment maintenance, especially in Grasberg open-pit mining," Papua-based union leader Virgo Solossa told Reuters.

"Production activities are still shut. We hope investigation teams complete their works as soon as possible."

Several investigations are being conducted into the collapse, including one by the Energy and Mineral Resources Ministry and one by Freeport Indonesia using international exports.