Rio had previously announced it may sell its Energy America unit, as well as uranium deposits in the US and Australia, in a bid to deter a hostile takeover from BHP Billiton.

Speaking to Bloomberg, Rio energy unit head Preston Chiaro said "expressions of interest" had been received for its coal mines.

Rio's operations in the US include the Antelope, Codero Rojo, Colowyo, Jacobs Ranch and Spring Creek mines.

BHP has until February 6 to make a formal bid for Rio. If it is successful, the combined company would become the biggest producer of thermal coal.