Shipments from its Australian mines fell to 81.9 million tonnes in the third quarter compared with 85.8 million in the same period a year ago, the miner said, snapping four consecutive quarters of increases.

UBS had estimated quarterly Pilbara iron ore shipments of 82.5 million tonnes.

In August, a truck operator was fatally injured at Rio's Paraburdoo Iron Ore mine in Western Australia.

Rio, which competes with BHP and Brazil's Vale SA in the seaborne-traded iron ore market, maintained its target to ship at the upper end of the guidance range of 330 to 340 million tonnes for 2018.

The Anglo-Australian said on Oct. 11 that it expected raw materials costs for its aluminium division about $400 million higher versus a year ago for 2018 and that costs should stay elevated into next year.

Higher thermal coal prices are likely to have an additional negative impact of $100 million in 2018 for its Pacific Aluminium smelters, it added.

Rio Tinto, which has contracts with sanctions-hit aluminium maker UC Rusal, continues to monitor the impact of the U.S. sanctions but has not declared force majeure to date, it said.

The weak quarter comes after a brisk first half, when all of the big four iron ore miners bolstered shipments to China which seeks higher grades to comply with stricter environmental regulations.

China buys just over half of Australia's commodity exports.

In an effort to maintain production capacity to feed China's giant steel industry Rio said on Oct. 1 that the miner and its Japanese joint venture partners will spend about $1.55 billion at two iron ore projects in Western Australia.

The announcement came as Rio has been selling off its coal assets in order to focus on its core commodities.

In other metals, Rio’s copper output jumped 32 percent to 159,700 tonnes in the quarter thanks to robust production from its Kennecott Utah Copper business.

(By Aditya Soni in Bengaluru; Editing by Sandra Maler)