Warrior Met Coal said an employee had died in an accident at one of its mines in Alabama.
The company said the incident occurred on Wednesday at its mine No. 4 in Brookwood, Alabama.
The United Mine Workers of America in a separate statement said the labor union was working closely with the US Mine Safety and Health Administration, the company, and other authorities to determine the cause of the incident.
In 2001, a disaster in the neighboring No. 5 mine had claimed the lives of 13 workers.