Four people were killed and seven more are missing following a gas explosion at a coal mine in southern Poland on Wednesday, officials said.

"One of the miners injured by the methane explosion at the Pniowek mine has died in hospital," the company in charge of the mine, JSW, said on Twitter.

"That is the fourth victim of today's mining disaster." 

Polish deputy prime minister Jacek Sasin tweeted that rescuers were still looking for seven other people. 

Polish Prime Minister Mateusz Morawiecki expressed his condolences and will travel to the area.

The company said earlier that 21 people had been injured and also spoke of two explosions.

It said a first blast occurred shortly after midnight at a depth of 1,000 metres (3,281 feet).

There were 42 miners in the area where the explosion happened and many of them suffered burns.

A second explosion occurred while rescue workers were assisting the victims of the first blast.

Poland, which still relies on coal for the majority of its power, has seen several other mining accidents in recent years.

Last year, two men were killed and two others injured when an underground wall collapsed at the southern Myslowice-Wesola mine. 

In 2018, five miners were killed in an earthquake that hit the Zofiowka mine, also in the south.