(REUTERS) -- An Indonesian workers' union plans to extend a three-month strike at Freeport McMoRan Copper & Gold's Grasberg mine until Jan. 15, union official Virgo Solossa told Reuters on Saturday.

The union, which represents about 8,000 miners at the world's second biggest copper mine, said they wanted to formally extend the strike for another month pending a deal with the Phoenix-based firm.

Last week we sent a letter to inform we are extending the strike until Jan. 15, Solossa said. The last strike extension is due to expire on Dec. 15.

However we hope that before the extension (is) realised, we can already have struck a deal with the management, because we want this problem be solved before Christmas, he said.

Many Freeport Indonesia workers, who are mostly Christians, are worried they cannot afford to celebrate Christmas due to the strike. Workers have been on strike over pay since mid-September.

The strike, the longest and most serious industrial dispute in Southeast Asia's top economy, has crippled the mine's production and cut Indonesia's tax revenue from its single biggest taxpayer.