The company said that the poor demand for metallurgical coal has led it to take the decision.

In a regulatory filing, Cliffs Natural said it issued Worker Adjustment and Retraining Notification (WARN) act notice to the employees, the United Mine Workers of America and state and local government agencies over the proposed plan.

Cliffs Natural Resources spokesman Ryan Thorn said: "The persistent depressed pricing and oversupply in the metallurgical coal market for the past year has put cost pressures on the business."

"Pricing is below the mine’s cash costs and at a point where it is becoming uneconomical to operate at these levels," Thorn said.

The company said it could idle the mine for over six months starting around 25 August 2014.

In 2013, the Pinnacle mine produced 2.8 million tons low volatile metallurgical coal and had a workforce of 552.