Dozens are believed trapped in ruined Amatrice, Accumoli and Pescara del Tronto, in mountainous central Italy.
Rescuers have advised journalists and bystanders to leave Amatrice urgently, as "the town is crumbling", the BBC's Jenny Hill says.
New cracks appeared in the town's hospital after strong aftershocks.
Officials revised down the number of dead after earlier giving a figure of 247.
The search for survivors went on through the night, amid hundreds of tremors and an aftershock which rocked already damaged buildings.
More than 4,300 rescuers are using heavy lifting equipment and their bare hands.
