“A doctor pronounced the death of two other miners… The tragic toll of yesterday’s tremor in the mine has risen to four,” JSW said in a statement after the rescuers reached two miners that were part of a group of four.

The bodies of the first two miners were found earlier on Sunday.

“The rescue workers will continue to look for the other six,” JSW said.

“The conditions in the affected zone are difficult, it added, underlining “the high concentrations of methane and the raised temperatures were effectively slowing down the rescue operation”.

The accident on Saturday – the second such incident in Poland last week – happened at the Zofiowka mine in the south.

Poland, which relies on coal for some 70 percent of its power, has had several other mining disasters in recent years.

A tremor shook the Zofiowka pit at 3:40 am (0140 GMT) on Saturday, 900 metres (2,950 feet) below the surface, sparking a methane leak.

All but 10 of the 52 miners underground managed to make it to the surface.

JSW is also the owner of a mine in Pniowek – six kilometres (3.7 miles) from Zofiowka – where an accident on Wednesday killed five, 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 hurt, including six with serious burns. The search for the seven people missing was called off on Friday after a third blast made conditions in the pit too dangerous, JSW said.