Rescuers were digging their way into Sago mine in the hope of finding the miners, said Steve Milligan of Upshur County's Office of Emergency Management on Monday.
"It could be hours, or it could be days," Milligan said. He said that the rescue was initially hampered by high gas concentrations outside the mine. Local rescuers were joined by others from neighbouring states of Virginia, Pennsylvania and Maryland.
The cause of the blast, which occurred about 6.30am (1130 GMT), was not yet known, officials said. There had been no communication with the trapped miners as darkness fell on the area.
Joe Manchin, the governor of West Virginia, said: "We're really hoping and praying for a speedy recovery and a safe recovery for them. We don't know what could have happened. It was just a horrific accident."
After ventilation fans dissipated the unidentified gas outside the mine, rescuers headed inside to assess damage and started digging through debris blocking access to the miners, Milligan said.
The mine, owned by International Coal Group Inc which acquired it through a recent merger, is in central West Virginia, about 160km from Charleston.
Miners are equipped with breathing equipment and other survival supplies.
"There's always that possibility, that hope and that chance, they were able to go to a part of the mine that still has safe air," Manchin said. "There's places they can retreat in all these mines. They have catacombs."
The explosion happened when the mine was reopening after it had been closed for the holidays, according to Manchin's spokeswoman, Lara Ramsburg.
Ramsburg said two cars had been entering the mine and the second car, carrying six miners, made it out after feeling the effects of the explosion.
The six miners tried to re-enter the mine to rescue their fellow workers but could not reach them, she said.
"We're really hoping and praying for a speedy recovery and a safe recovery for them"
Gas levels outside the mine had threatened further explosions and caused officials to evacuate the area before the gas was dissipated. The gas was initially thought to be methane, but officials later said they were not certain what it was.
Since October, the US Mine Safety and Health Administration has issued 50 citations to Sago mine, some as recently as on 21 December, including citations for accumulation of combustible materials such as coal dust and loose coal.
In 2002, nine coal miners in Pennsylvania were rescued after being trapped for 77 hours in a flooded shaft 240ft underground.