The readings were made about 7.30pm on July 15, prompting concern that a fire was causing the production of the lethal gas.

“All underground employees were safely evacuated and none sustained injuries.” New York-listed Foresight said.

“Hillsboro officials identified the affected area and have been working with federal and state officials implementing an approved plan to contain and extinguish. Two holes have been drilled into the coal mine and water is being pumped into the affected area.”

San Franciso-based Citigroup analyst Brian Yu told Bloomberg the extent of the damage won’t be clear until carbon monoxide levels return to normal and it’s deemed safe to re-enter.

“The company’s coal shipments shouldn’t be affected in the near term because it has sufficient stockpiles and excess capacity at its nearby Macoupin complex,” the newswire reported Yu as saying a research note.

“The fire occurred in an area far enough from the company’s longwall mining equipment and the coal face that neither will likely be damaged, Yu said.”

Foresight, announced the news of the fire incident on Thursday – causing its shares to slide 3% to close at $US19.56 that day. The coal company floated its shares last month at $20.00 each.

The underground thermal coal mine typically produces about 500,000 tons per month.