Rescuers were trying to reach 10 miners who went missing today after a second coal mine accident in Poland this week.
Poland, which relies on coal for some 70% of its power, has had several other mining disasters in recent years.
The latest incident took place at the Zofiowka mine in the south of the country.
A tremor shook the pit at 3:40am local time, 900 metres below the surface, sparking a methane leak, its owner JSW said.
All but 10 of the 52 miners underground managed to make it to the surface, the company said.
"Sadly there is a strong possibility that there have been deaths," said Polish President Andrzej Duda, who visited the site this evening.
Prime Minister Mateusz Morawiecki, who also visited the scene today, called the accident "devastating".
In 2018, five miners were killed when an earthquake struck the same coalmine.
JSW is also the owner of a mine in Pniowek some 230 kilometres south, where an accident on Wednesday killed five people, including a rescuer, and left seven missing.
An initial explosion occurred shortly after midnight on Wednesday at a depth of 1,000 metres.
A second shook the site while rescue workers were helping the victims of the first.
Twenty people were injured, including six with serious burns. The search for the seven people missing was called off yesterday after a third blast made conditions in the pit too dangerous, JSW said.
Morawiecki stressed that both recent accidents would be "checked very carefully" to determine whether they were simple accidents due to natural causes or whether mistakes had been made.
In addition to the 2018 accident in Zofiowka, last year two men were killed and two others injured when an underground wall collapsed at the southern Myslowice-Wesola mine.
