The Colorado-based, Powder River Basin-focused miner confirmed that it controlled more than 1.1Bt.

It was aided by Westmoreland’s acquisitions this year and last, including the addition of tonnage neighboring its Absaloka and Rosebud mines.

At Absaloka, officials just leased 145 million tons of Rosebud McKay coal resources from the Crow Tribe of Indians and its Rosebud mine leased 56.4Mt of resources last year from a private landowner.

In all, Westmoreland's resources comprise about 553Mt of proven and probable reserves and an additional 625Mt of resources that it said would require additional drilling to meet US Securities and Exchange Commission requirements before being classified as reserves.

“Securing adequate resources to meet our long-term customer commitments has been a high priority,” chief executive officer Robert King said.

“I am pleased that we have achieved this significant milestone and solidified our resource position."

Between its lease agreement and the extension of a long-term supply deal it signed in late January, Westmoreland is enjoying a busy year of growth.

In February the company marked its success with a new office for its operational employees in Billings, Montana.

Westmoreland’s holdings include coal in the PRB in Montana, sub-bituminous mining in Wyoming and lignite mining in Montana, North Dakota and Texas.