April 24 (Reuters) - Cliffs Natural Resources Inc, which is facing off against an activist investor who wants to break up the company, reported a first-quarter loss on Thursday on the back of lower market prices for iron ore and metallurgical coal.

The U.S.-based miner reported a net loss of $83 million, or 54 cents a share, in the three months to end-March from a net profit of $97 million, or 66 cents a share, in the same period a year earlier.

(Reporting by Nicole Mordant in Vancouver; editing by Matthew Lewis)