The number was increased by authorities to 20, following a more detailed identification of all workers sent to the mine.
The blast, caused by gases released in the fire, occurred in an old shaft about 330 meters below the surface, Cicmanec said.
He repeatedly declined to speculate on the miners' chances for survival pending further efforts to save them.
He said visibility was slowly improving and rescue teams were down in the mine, in a corridor leading to the epicenter of the explosion.
Prime Minister Robert Fico and Interior Minister Robert Kalinak traveled to the mine about 125 miles (200 kilometers) northeast of the capital, Bratislava.
The trapped workers include nine miners who were initially sent to battle the blaze, and 11 sent as reinforcements as the fire grew. Mine managers said they lost all contact with the 20 men after the blast.
The explosion is attributed to the buildup of flammable gases, including carbon monoxide, "produced by imperfect burning" of brown coal.
Nine other people who also were in the mine Monday but not near the explosion were treated for minor injuries in a hospital and discharged.
Four miners died in 2006 in the nearby mine of Novaky after a shaft caved in.
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