Two floors of the small four-storey block in the town of Torre Annunziata, at the foot of the volcano Mount Vesuvius, collapsed at around 6.30 in the morning (0430 GMT).
Media reports said neighbours had heard no explosion -- just the crash when the building collapsed.
Witnesses said a freight train had passed on the neighbouring railway track just before the building collapsed, while others noted that renovation works had been under way on the lower floors.
Prosecutors in Torre Annunziata have opened an investigation to determine what caused the accident.
Some 30 firefighters, joined by dozens of volunteers and relatives of the missing, were searching the wreckage for survivors in the scorching heat.
The fire service said two officers had been lightly injured during search operations.
Local mayor Vincenzo Ascione said the missing were a couple and their children aged eight and 11, another couple and their 25-year-old son, and a 65-year-old woman who lived alone.
The rescuers were stopping regularly, hoping to hear calls for help. Sniffer dogs were also on site to help locate the missing.
Train traffic on the adjacent railway track was halted as a safety precaution.
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