A further 10 miners were injured in the explosion Thursday afternoon at the CSM mine near the town of Karvina. Polish President Andrzej Duda declared Sunday a day of national mourning for the victims of the tragedy. Flags in Poland will be lowered to half-staff on public buildings and large sporting and entertainment events will be canceled.
Polish Prime Minister Mateusz Morawiecki and his Czech counterpart Andrej Babis have offered their condolences to the families of the victims. The two leaders visited the mine on Friday. "To our knowledge ... there is a fire underground, very high temperature, very high risk of subsequent explosions," Morawiecki said.
The Polish leader visited two injured miners at the University Hospital in the nearby city of Ostrava. One of them was in critical condition with burns over 50 percent of his body, hospital spokeswoman Nada Chattova said.
Another miner was release after being treated in Karvina. "I wish to express words of deepest sympathy to all the close victims of the mining disaster in Karvina," Morawiecki said. "This is a huge tragedy for all Poles and Czechs. On this difficult day, we strongly show our solidarity and sense of national community."
The explosion occurred about 800 meters (2,600 feet) underground. OKD executive director Boleslav Kowalczyk said efforts to recover the bodies were continuing on Friday despite a fire in the mine. Authorities have been investigating the accident.
Bohuslav Machek, spokesman for the Czech mining authority said the level of methane in the mine was at least 4.5 times the allowed level at the time of the explosion. Easily ignitable methane is naturally present at black coal mines, posing a threat for miners.
A previous version of this story was corrected to show that the mine is called CSM, not CSA.
Vanessa Gera in Warsaw contributed to this report.