Coal miners have seen better days. In this photo, workers look on as Republican presidential candidate Mitt Romney speaks during a campaign rally at Murray Energy-owned American Energy Corporation on August 14, 2012 in Beallsville, Ohio. GETTY/Justin Sullivan

Murray Energy, the United States' largest underground coal company, is preparing to lay off hundreds of workers, according to the miners' labor union. Such a move would be the latest sign of a struggling industry, which is coming off what is likely one of its least productive years in the U.S. in recent memory.

The layoffs will hit at least 690 workers -- or roughly 9 percent of the company's workforce -- at five West Virginia mines represented by the United Mine Workers of America, the union told International Business Times. It remains unclear how many workers at the company’s non-union mines will be affected.

The Ohio-based company, which is privately held, declined to go into specifics about the job losses. But it confirmed layoffs will take place and blamed them, in part, on the Obama administration, which it has sued over environmental regulations.

“Murray Energy Corporation confirms that certain workforce adjustments and scheduling changes, made in the normal course of business, are necessary to reflect the current coal marketplace, which has been destroyed by President Barack Obama and his supporters, including the Sierra Club, the increased utilization of natural gas to generate electricity, and the extremely excessive coal severance tax in the state of West Virginia,” Murray Energy spokesman Gary Broadbent said.

Coal Production in the United States | FindTheData

Murray Energy already laid off more than 1,800 workers earlier this year.

“This is one last, cruel blow delivered to coal miners and their families in 2015 by the continuing weak coal market,” union spokesman Phil Smith said. “We look forward to an improving market in 2016 so that these workers can return to their jobs as soon as possible.”

As the nation’s power plants increasingly turn to cheap natural gas, American coal producers have struggled. The past year has seen a number of high-profile companies seek bankruptcy protection and job cuts. On Wednesday, nearly 2,000 miles away from Appalachia, Montana-based Signal Peak Energy announced it is slashing coal production and laying off 58 workers. And on Tuesday Walter Energy laid off over 300 workers at a mine in Alabama.

Alpha Natural Resources, Patriot Coal and Walter Energy have all entered bankruptcy proceedings this year. Earlier this week, a federal judge granted Walter Energy the ability to throw out its collective bargaining agreement and pull the plug on its health and pension obligations to nearly 3,000 miners and their dependents.

Overall employment in coal mining has declined from about 73,000 to roughly 65,000 over the last year, according to the Bureau of Labor Statistics.

Murray Energy CEO Bob Murray has maintained his company is prepared to ride out the hard times. At a world coal conference in October, he predicted coal’s share of the U.S. energy mix will fall to 30 percent by 2020, down from its current level of 37 percent. “I want to be the low-cost producer of that market that’s left,” he said at the time.

Murray Energy has tussled with federal regulators. It recently attempted to depose Environmental Protection Agency Administrator Gina McCarthy as part of a lawsuit over the agency’s new rules on carbon pollution from power plants.

Last month, the company was fined $150,000 by the Mine Safety and Health Administration for violating workers’ rights to make confidential safety complaints to the federal government at five union-represented mines. CEO Bob Murray was ordered to personally deliver speeches at the mines informing workers of their rights. The company has appealed the ruling.