Citing Spanish news outlet El Pais, the Associated Press said the incident occurred at the Santa Lucia operation near Pola de Gordon.
The mine is operated by Hullera Vasco Leonesa.
The Castilla and Leon regional emergency services told the AP that the injured miners were taken to a Leon medical center and that a medical helicopter and three ambulances were dispatched to the mine Monday morning.
It is believed the six miners killed succumbed to gas inhalation while working about 700 meters underground.
El Pais reported that emergency responders evacuated five other neighboring mines. Ten individuals were believed to still be trapped underground mid-day Monday.
About 40 mines operate in that region of the country, staffed by about 8000 miners.
