US mining giant Freeport-McMoRan said it will resume underground operations at its site in eastern Indonesia on Wednesday, nearly two months after a tunnel collapse killed 28 workers.

The announcement on Tuesday came after the energy ministry gave the green light for the firm's Indonesian subsidiary to restart underground production at Grasberg, its huge gold and copper mine high in the mountains of Papua province.

"Today we received permission to resume production and processing at our underground mine," said Daisy Primayanti, spokeswoman for the firm's Indonesian unit.

Underground operations will resume on Wednesday, she said. Open-pit mining and milling operations resumed at the end of June.

Production at Grasberg was halted entirely the day after the May 14 tunnel collapse, which killed 28 workers as they took part in a safety training course and which was one of Indonesia's worst mining accidents.

Freeport declared force majeure on shipments from Grasberg in June, declaring it could not fulfil contractual obligations due to circumstances beyond its control.

Primayanti said Tuesday the clause would stay in place for the time being.

Last month the company suspended four senior employees, including three foreigners, whom staff accused of being at fault over the tunnel collapse.

In 2011 a three-month strike crippled production at the mine, and workers only halted the industrial action once Freeport agreed to a huge pay rise.

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