WARSAW — An explosion in a coal mine in the Czech Republic killed at least 13 people on Thursday, the nation’s deadliest mining disaster in nearly three decades.

The blast, caused by methane gas igniting, devastated large portions of a hard-coal mine near Karvina, a small city on the border with Poland, according to the state-run OKD mining company.

The number of people injured and the extent of the damage were not immediately clear, with rescue workers struggling to reach the site of the blast, more than half a mile underground, because of flames and smoke, according to company officials speaking on Czech television.

“Due to yesterday’s mine disaster, 13 miners lost their lives,” a company spokesman, Ivo Celechovsky, said at a news conference. Twelve of the dead miners were Poles, and one was Czech.