It’s believed as many seven people could be trapped inside the ruins of the block, which is near the city of Naples.
According to first reports two families could be among the missing.
Italian firefighters shared a shocking image from the scene and authorities are working to find anyone who may have been trapped.
Firefighters told Sky TG24 that six or seven people are believed to be buried in the rubble in the town of Torre Annunziata.
Witnesses said that there was no explosion before the collapse Friday morning but that a train had just passed by on adjacent train tracks, according to news agency ANSA.
Firefighters on long, retractable ladders were checking the stability of an adjacent building in the residential neighborhood.
Images show the structure partially collapsed, revealing the interior of some apartments.
Many rescue workers were removing rubble by hand, passing it out in buckets.
News reports indicated that reconstruction work was being carried out on the building.
Torre Annunziata is around four kilometers (three miles) from the Pompeii archaeological site.