Rescue workers have searched through tonnes of concrete and steel from the collapsed Genoa highway bridge for a fourth day as Italians prepared to bury the 38 victims of a disaster that has angered the public and rattled industry.

A fire among the rubble held up part of the operation on Friday, sending clouds of white smoke up before being extinguished.

A spark from metal-cutting equipment was thought to have caused the blaze that started in a warehouse under the ruined bridge.

Italian bridge collapse: Genoa disaster in pictures View Gallery

The confirmed death toll from the collapse of a 200m stretch of the bridge on Tuesday, which sent vehicles plummeting 50m to the ground during a downpour, remained at 38 but officials say there still may be 10-20 people missing.

The search entered a new phase on Friday as heavy equipment started moving large sections of the bridge in order to clear new areas to probe.

A state funeral for most of the victims is due to be held on Saturday morning, led by Genoa's archbishop, Cardinal Angelo Bagnasco, and attended by President Sergio Mattarella and Prime Minister Giuseppe Conte.

But some families say they will boycott the event and hold their own private services in protest against what they say was negligence that caused the bridge to collapse.

The government has declared Saturday a national day of mourning.

Shares in the parent company of Autostrade per l'Italia, which runs the highway plunged more than 30 per cent in the days after the collapse, but were recovering slightly on Friday as investors said government threats to revoke its concessions might be political rhetoric rather than a likely outcome.

The transport ministry has given Autostrade 15 days to show it had met all its contractual obligations and wants the company to rebuild the bridge at its own expense.