The death toll from an underground explosion at a Polish coal mine last week rose to 17 on Wednesday, and Pope Benedict XVI expressed his condolences for the victims.

Two miners, one 40 years old and the other 21, became the latest victims when they died of severe burns, hospital spokesman Miroslaw Rusecki said.

The explosion 3,450 feet below ground on Friday was caused by a methane leak at the Wujek-Slask mine in Ruda Slaska. Twelve miners were killed immediately, and the death toll has crept higher since then as injured miners have succumbed to burns.

Authorities have opened an investigation that will try to determine whether all security precautions were being enforced at the mine.

Pope Benedict XVI expressed his condolences for the victims of the explosion.

"God protect all workers from such tragedies," Benedict told the faithful during a general audience at the Vatican.

Poland remains highly dependent on coal as an energy source, with about 95 percent of the country's electricity still derived from it.

Mines dot the landscape and there have been several mining tragedies over recent years.

Friday's blast was the most serious since a methane gas explosion killed 23 miners in November 2006 at the Halemba mine, a different pit in the same city, Ruda Slaska.