About 30 people were killed on Tuesday when a giant motorway bridge collapsed in heavy rain in the Italian city of Genoa in what the government called an “immense tragedy”.

The collapse, which saw a vast stretch of the A10 freeway tumble on to railway lines in the northern port city, came as the bridge was undergoing maintenance work and as the Liguria region, where Genoa is situated, experienced torrential rainfall.

“Unfortunately there are around 30 dead and many injured in a serious condition,” Interior Minister Matteo Salvini told reporters.

Rescuers scouring through the wreckage, strewn among shrubland and train tracks, said there were “dozens” of victims, as rescue helicopters winched survivors on stretchers from the ruined bridge.

Cars and trucks were tangled in the rubble and nearby buildings damaged by vast chunks of concrete, according to an AFP photographer at the scene.

“We're not giving up hope, we've already saved a dozen people from under the rubble,” a fire official, Emanuele Giffi, told AFP.

“We're going to work round the clock until the last victim is secured.”

The incident — the deadliest of its kind in Europe since 2001 — is the latest in a string of bridge collapses in Italy, a country prone to damage from seismic activity but where infrastructure generally is showing the effects of a faltering economy.

'Unacceptable to die like this'

Aerial footage showed more than 200 metres (650 feet) of the viaduct, known locally as the Morandi bridge, completely destroyed.

“I'm following with the utmost apprehension what is happening in Genoa and what looks like it could be an immense tragedy,” Transport and Infrastructure Minister Danilo Toninelli said on Twitter.

Salvini, who is also leader of the nationalist League party in a power-sharing government, vowed to hold those responsible for the disaster accountable.

“I have gone over this bridge hundreds of times, and I commit to digging and finding out who is responsible for an unacceptable tragedy, because it's not possible that in 2018 you can work and die in these conditions,” he said.

The cause of the disaster was not immediately clear, although weather services in the Liguria region where Genoa is situated had issued a storm warning on Tuesday morning.

The national motorways body said on its website that “maintenance works were being carried out on the base of the viaduct”, adding that a crane had been moved on site to assist the work.

Shares in Italian company Atlantia, which runs much of Italy's motorway network including the collapsed stretch of the A10, closed the day down more than five per cent at 23.54 euros.

History of collapses

The mayor of Nice, 200 kilometres (125 miles) west along the coast, said he had authorised firefighters in the French city to assist their counterparts in Genoa.

Genoa, home to half a million people, is located between the sea and the mountains of northwestern Italy.

Its rugged terrain means that motorways that run through the city and the surrounding area are characterised by long viaducts and tunnels.

The Morandi viaduct, completed in 1967, spans dozens of railway lines as well as an industrial zone housing several factories.

One factory, immediately next to one of the viaduct's support columns, was virtually empty on Tuesday the day before a national holiday, and seems to have sustained minimal damage.

“I live nearby and I cross the bridge every day on foot,” said Ibou Toure, 23, a translator. “I was never sure of it, you'd always hear these noises whenever lorries were going over.

“When I heard it had collapsed, I wasn't surprised.”

In March last year, a couple were killed when a motorway overpass collapsed on their car near Ancona on the country's Adriatic coast.

A pensioner died in October 2016 when his car was crushed by a collapsing bridge over a dual carriageway between Milan and Lecco.

That incident was blamed on bureaucratic bungling which led to a fatal delay in the bridge being closed after it was reported to be showing significant cracks.