SYDNEY--Standard & Poor's Ratings Services cut Fortescue Metals Group Ltd.'s corporate credit rating, cautioning the Australian miner was facing considerable challenges from a sharp downturn in the iron-ore market.

Fortescue--the world's fourth-largest iron ore exporter by volume--has been scrambling to rein in spending as it grapples with tumbling iron ore prices, which have fallen more than 60% since the start of last year.

On Wednesday, S&P lowered Fortescue Metals Group's credit rating to BB from BB+, with a negative outlook. It meanwhile lowered ratings on its senior secured debt to BBB- from BBB, and on its unsecured debt to BB- from BB.

"The downgrades reflect our expectation that Fortescue's financial risk profile will weaken significantly in the next two years due to low iron-ore prices," said analyst May Zhong.

Fortescue last week faced downgrades from rival ratings firm Moody's, which warned of a "substantial weakening" in store for the company's earnings and credit.

Executives have sought to reassure investors the Australian miner can survive the sharp downturn in prices, this month reporting sharply lower production costs and saying Fortescue continues to generate a profit from its mines.

Ms. Zhong said S&P's new BB rating was reliant on the company's ability to drive down costs and keep its operations profitable. "Fortescue's credit metrics could fall below our current expectation if there is any misstep in reducing its cash production cost or there is any unfavorable foreign exchange movement," she said.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com

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