Warsaw: Suspected methane blasts have killed at least four people and injured 20 others in a coal mine in southern Poland, authorities said on Wednesday.

Mining company JSW said 13 teams were involved in a rescue operation to find several workers who are still missing. However, efforts had to be put on hold due to difficult conditions underground.

“The rescue operation was temporarily suspended until the dust barrier, protecting the rescue base against the methane explosion, is rebuilt,” the company said in a statement.

JSW said the explosions occurred at its Pniowek mine in Pawlowice shortly after midnight at a depth of 1,000 metres (3,281 feet).

There were 42 miners in the area at the time. Most of the injured suffered from burns, doctors told reporters.

Polish Prime Minister Mateusz Morawiecki expressed his condolences on Twitter, saying his “thoughts and prayers” were with the miners’ families and that he would travel to the area.