BHP Group (LSE: BHP) on Thursday reported mixed production numbers for the first half of its financial year, with output of some key commodities like copper and iron ore declining from last year.

The mining giant warned of particular issues at its nickel and metallurgical coal operations even as production at some other assets improved. It left most of its full-year guidance unchanged.

Copper production for the half-year period rose 7.2% to 894,000 tonnes. However, output in the second quarter declined 4% sequentially to 437,400 tonnes.

Iron ore output fell 2.3% to 129 million tonnes in the first six months.

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BHP sharply cut its metallurgical coal production forecast for the full fiscal year to 23-25 million tonnes, from 46-50 million tonnes previously. This comes after a fatal incident earlier this week at its Saraji coal mine in Australia.

The company said it was evaluating options to mitigate the impact from a “sharp fall” in nickel prices recently. Nickel output rose 4% annually in the first half to 40,000 tonnes.

BHP’s shares were unchanged on Thursday morning following the operational update.