Rating agency Fitch downgraded its outlook on BHP Billiton and Rio Tinto from stable to negative, after revising down its price assumptions for iron ore, copper and nickel earlier this month.

BHP Billiton, the world's largest mining firm, held its A+ rating but Fitch said on Wednesday the spin-off last month of some of its assets into a new company named South32 would have a marginally negative effect on its credit rating in the near term, weighing on projected free cashflow generation.

The outlook downgrade on A- rated Rio Tinto, the world's second-largest mining firm, was on the back of weaker price expectations for iron ore, its main product.

"Although Rio Tinto benefits from a leading iron ore cost position, the high percentage of revenue and (earnings) generated by that single commodity exposes the company to significant risks," Fitch said in a statement.

The rating agency confirmed its negative outlook on BBB- rated group Anglo American.

Despite some positive developments such as the delivery of first production from its large Minas Rio iron ore project in Brazil, and a quick ramp up at its platinum mines in South Africa after a protracted mining strike last year, high debt remained an issue for the firm.

"Elevated leverage, which was driven by the intensive capital spending of previous years remains a primary risk, in our view, and is reflected in the negative outlook," Fitch said.