The explosion occurred just before 2300 local time (2100 GMT) on Wednesday, 900 metres underground in the Borynia mine at Jastrzebie Zdroj, near the Czech border.
In total, there were 32 miners carrying out maintenance work when the blast happened, five of whom escaped unhurt.
Common occurence
Jablonska-Bajer said it was not clear what had set off the explosion, because there was no obvious source for an electric shock, which is often to blame for methane blasts.
She said: "The cause was a methane explosion, but we can't say how the explosion happened. None of the methane sensors were showing high levels of methane."
A similar explosion in January killed two miners at another mine in Silesia.
The mining industry is crucial in Poland, which is the European Union's top producer of coal.
About 96 per cent of Poland's electricity is generated in coal-fired power stations.
For families and colleagues of the victims, the explosion was a reminder of the dangers faced daily in Polish pits.
One miner, Tomasz Lisiecki, said: "All the time I've thought that this could happen to me as well as to my colleagues. But you have to work somewhere."
Accidents in Polish mines are not uncommon, with a total of 15 miners dying in underground accidents last year.
In November 2006, 23 miners were killed in a gas explosion at the Halemba coal mine in Silesia, in Poland's worst such accident in 30 years.