Thirteen coal miners are trapped following an explosion in a mine shaft in the US state of West Virginia.
Friends and relatives face an agonising wait for news
The blast happened about one mile (1.6km) underground at the Sago Mine in Buckhannon between 0600 and 0630 (1100 and 1130 GMT), local officials said.
Initial attempts to reach the miners were abandoned, but small teams using camera and audio equipment are now inside the mine testing air quality.
Six other men were reported to have escaped from the mine unharmed.
A senior emergency management official in Upshur County, West Virginia, said that immediate rescue efforts had "hit a wall" after a build-up of gas.
But rescuers returned to the mine after the gas had been pumped out, and are trying to determine the men's location.
The operation is expected to last several hours, with a number of teams working in shifts throughout the night.
Hundreds of friends and relatives have gathered at a nearby Baptist church for an agonising wait for news.
West Virginia governor Joe Manchin said that as long as the miners had survived the initial explosion, they could remain underground for some time.
"There's places they can retreat in all these mines. They have catacombs," he told CNN.
The miners have basic food and water, and are equipped with individual air purifying systems that would give them up to seven hours of clean air, officials said.
Terry Farley, of the state Office of Miners' Health Safety and Training, told AP there was no immediate explanation for the blast.
However, Mr Manchin's spokeswoman, Lara Ramsburg, said it may have been triggered by lightening, as the explosion happened at the same time as heavy thunderstorms in the area.
International Coal Group recently took control of the Sago Mine, located in northern West Virginia, about 100 miles (160km) from the state capital, Charleston.