Italy's fire service said at around 3:15pm that there had at that point been no signs of life, but that rescuers continued to work "with the objective of finding people alive".
Teams of sniffer dogs and 80 firefighters were involved in the search, together with volunteers and relatives of the families who lived in the collapsed building, some sifting through the rubble by hand.
USAR (Urban search and rescue teams) from Lazio and Tuscany joined the search effort but said on Friday afternoon they had received "no response" after using geophones to try to locate any victims.
The affected building. Photo: Vigili del Fuoco
Two firefighters and one rescue dog were lightly injured during the search effort.
The four-storey block of flats was located in in Torre Annunziata, a town at the foot of Mount Vesuvius in the Campania region, marked on the map below. The building's third and fourth floors - where the two families feared missing lived - completely gave way at around 6:35am on Friday.
The Circumvesuviana train line which passes near the building, travelling between Naples and Pompeii, was suspended on Friday morning to allow the search effort to continue and due to debris on the tracks.
Torre Annunziata has seen several building collapses in recent years. In August 2014, two people were injured after an apartment building collapsed, and it later emerged that local authorities had issued repeated orders to the building's private owners to carry out work to secure the building since 2011.