Global credit ratings agency Fitch has handed negative ratings outlooks to BHP Billiton, Rio Tinto and Anglo American.

Sliding prices for iron ore, copper and nickel are behind the darkened view, which has seen the credit rating of all three companies reaffirmed, but the possibility of future downgrades raised.

Fitch forecasts iron ore prices at an average $US50 a tonne this year and next, before rising to $US60 in 2017 and $US70 after that.

The current iron ore spot price is at around $US65.10 a tonne, much better than the mid-40s it slipped to in early April.

But Fitch assumes BHP Billiton and Rio Tinto will both reduce the rate of capacity increase in the Pilbara, which will lead to lower development costs.

BHP Billiton is the best rated of the three firms at A+, due to superior diversification.

The rating remains despite the company spinning off some of its assets into a new company called South32.

Rio Tinto retains its A- rating, though analysts predict it will be free cash flow negative over the next couple of years, unless the iron price improves dramatically.

Anglo American stays at BBB, and continues to hold more debt relative to its earnings than the two Anglo-Australian firms.