Rescuers mounted a massive effort on Monday to reach 13 miners trapped deep inside a West Virginia coal mine after an early morning explosion.

The men became trapped inside the Sago mine in central West Virginia, about 100 miles from Charleston, after an unexplained blast at about 6:30 a.m. (1130 GMT) and rescue efforts have been hampered by high gas concentrations outside the mine. There had been no communication with the miners since.

After ventilation fans dissipated the unidentified gas outside the mine, two teams of rescuers began advancing through the shaft to reach the men 250 feet below ground.

The first team had gone through 1,500 feet and had not encountered any debris, said Roger Nicholson, General Counsel International Coal Group Inc., which owns the mine.

Another company official said that every 500 feet the rescue teams had to stop to test air quality and hook up a rescue telephone to the mine's shaft telephone lines.

"It's a very slow, careful process," said Gene Kitts, of ICG Mining Services.

Nicholson said another team was due to start drilling at around 9 p.m. (0200 GMT on Tuesday) down to the chamber in which the miners were thought to be trapped. They were hoping to test oxygen levels and drop a microphone down to listen for signs of life.

"We're really hoping and praying for a speedy recovery and a safe recovery for them," Gov. Joe Manchin told CNN. "We don't know what could have happened. It was just a horrific accident."

'HOURS OR DAYS'

Hundreds of family and friends gathered at a nearby Baptist church where the Red Cross had set up operations.

"It could be hours, or it could be days," said Steve Milligan of Upshur County's Office of Emergency Management. Local rescuers were joined by others from the neighboring states of Virginia, Pennsylvania and Maryland.

The explosion happened when the mine was reopening after being 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 reenter the mine to rescue their fellow workers but could not reach them, she said.

Gas levels outside the mine had threatened further explosions and caused officials to evacuate the area before the gas dissipated. The gas was initially thought to be methane, but officials later said they were not certain what it was.

There were also worries about a build-up of carbon monoxide from fires in the mine, said Terry Farley of the West Virginia Office of Miners' Health Safety and Training.

Since October, the U.S. Mine Safety and Health Administration has issued 50 citations to Sago mine, some as recently as December 21, including citations for accumulation of combustible materials such as coal dust and loose coal.

"The incident coincided with a local thunderstorm but we do not know at this time whether those events were related," said International Coal Group spokesman Roger Nicholson.

PHOTO CAPTION

Workers with a well drilling rig stand by as the ground is cleared near Sago, West Virginia January 3, 2006, two miles away from the entrance to the mine, where 13 miners, trapped following an explosion earlier in the day, were suspected to be. (REUTERS)