Rio Tinto on Friday reduced its 2021 iron ore shipments forecast, as a tighter labour market in Western Australia delayed the completion of a new greenfield mine at Gudai-Darri.

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The miner now expects 2021 Pilbara iron ore shipments between 320 million tonnes (mt) and 325 mt, down from the lower end of a previous range of 325 mt to 340 mt.

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The downgrade puts Rio on course to lose its spot as the world's biggest iron ore producer to Brazilian rival Vale S.A..

A tight global supply chain also added to its troubles, the company said.

"There are some clear headwinds from recent regulatory tightening (in China) and the transition may lead to some near-term volatility," Rio said.

Iron ore prices have nearly halved since hitting a record peak in mid-May, with demand hurt by China's steel output curbs and a sharp slowdown in the country's property activity due to a regulatory crackdown.

Still, Rio Tinto shipped 83.4 mt of the steel-making commodity in the three months ended September 30, 2 percent higher than the 82.1 mt shipped last year.