Freeport-McMoRan Copper & Gold's Indonesian unit has restarted concentrate shipments after a three-month strike ended in December, an official said on Wednesday, a sign that work at the world's second largest copper mine was returning to normal.

Copper smelter PT Smelting, which is part-owned by Freeport, is again receiving copper concentrate from the mine, Dwi Bagus, an assistant manager at the smelter, told Reuters by email.

Bagus declined to say when shipments restarted or the quantity of concentrate the smelter received, but a source at Amamapare port in Papua province said Freeport Indonesia shipped 27,000 tonnes of concentrate to PT Smelting in December.

The source at the port, which is near the Grasberg mine, also said cargo vessels containing 26,590 tonnes of Freeport copper concentrate left for east Java on Wednesday.

Officials in Freeport Indonesia were not immediately available for comment.

According to its website, Freeport Indonesia's copper concentrates are sold under long-term contracts, with approximately half the amount sold to affiliated smelters Atlantic Copper and PT Smelting.

Arizona-based Freeport has had a force majeure in place since October on concentrate exports from Grasberg, which also has the world's largest gold reserves and also produces silver, after workers went on strike for better pay.

Losses due to the strike amounted to 100,000 tonnes of copper concentrate, according to analysts estimates, and the stoppage triggered a spate of similar strikes in Southeast Asia's biggest economy.

Earlier this month, Freeport Indonesia said it continued to ramp up production at Grasberg.

Benchmark three-month copper on the London Metal Exchange traded little changed at $8,237.25 a tonne by 1024 GMT, versus $8,200 at the close on Tuesday.