The explosion on Sunday morning destroyed four of 18 apartments in the building, PAP said, quoting a local fire brigade representative.
The prosecutor's office said an investigation is expected to start on Monday.
"Looking at the degree of destruction of this building, it was initially assumed that it could have been a gas explosion, but we do not have certainty yet. For now, our activities are focused on the rescue operation," a state fire service spokesman told broadcaster TVN24.
Many households in Poland use gas for cooking or heating water.
(Reporting by Agnieszka Barteczko; Editing by David Goodman)