Three people have been pulled out of the rubble alive and taken to hospital, sources said.
Naples Prefect Claudio Palomba told reporters that no one was unaccounted for after initial fears people may have been killed by the collapse.
None of the injured people are in a life-threatening condition, the sources said.
A 19-year-old woman who suffered broken bones in many parts of her body was the person most badly injured.
Five crews of firefighters are at work on the scene.
A local woman said the building in question had visible signs on the facade that it had stability problems.
Prime Minister Giorgia Meloni on Sunday spoke to the commander of the fire brigade after the building collapsed and "is following the rescue operations", government sources said.
The sources said Meloni was being "constantly updated" about the situation.
ANSA
