A methane gas explosion in a coal mine in the northeast of the Czech Republic has killed 13 miners, the mining company said on Friday.

OKD, the company that operates the CSM mine near the city of Karvina, said 11 of those killed were Polish miners provided by a contractor agency, while two were Czech.

“Due to yesterday’s mine disaster, 13 miners lost their lives — 11 Poles and two Czechs,” OKD spokesman Ivo Celechovsky said in a televised statement.

A further 10 miners were injured in the blast, with one in a critical condition, the Czech national news agency CTK reported.

OKD’s managing director, Boleslav Kowalczyk, said search and rescue operations had to be called off because of poor conditions in the mine.

“Unfortunately, we have reached a point where it was impossible to move forward because there was a fire raging and zero visibility,” he said in a statement.

Czech Prime Minister Andrej Babis and his Polish counterpart, Mateusz Morawiecki, traveled to the mine Friday.

“I wish to express words of deepest sympathy to all the close victims of the mining disaster in Karvina,” Morawiecki said on Twitter. “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 in the CSM mine is a tremendous tragedy. I would like to express my deepest condolences to the survivors,” Babis added on Twitter.

The Czech leader also said he would assess what assistance his government can provide during his visit to the site on Friday.

The explosion occurred 800 meters (2,625 feet) below the surface just after 5 p.m. local time on Thursday. All underground work at the hard-coal mine was immediately halted and workers were evacuated, the company said.

The disaster is the worst mining accident to occur in the Czech Republic since 1990, when 30 miners died in a fire in another mine near Karvina, CTK reported.