Rome: A bridge on a main highway linking Italy with France collapsed on Tuesday in the Italian port city of Genoa during a sudden, violent storm, sending vehicles plunging 90 metres into a heap of rubble below.

At least 30 people have been killed, with the death toll expected to rise, Italy's Deputy Prime Minister Matteo Salvini said. Several politicians said the accident highlighted "years of potential maintenance postponed and shelved" for Italy's infrastructure due to budget constraints.

A section of the bridge crashed down over a river, an industrial zone, some railroad tracks and buildings in torrential rain. It collapsed at about 11.30am local time (7.30pm AEST), the local fire brigade said.

Up to 35 cars and at least three trucks were on the 80-metre section of the span that collapsed, said Angelo Borrelli, the head of Italy's civil protection agency.

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Hundreds of firefighters and emergency officials were searching for survivors in the rubble with heavy equipment. At least four people were pulled alive from vehicles under the bridge, the Italian news agency ANSA reported.

One unidentified woman who was standing below told RAI state TV the bridge crumbled as if it were a mound of baking flour. Video of the collapse captured a man screaming: "Oh, God! Oh, God!"

Other images showed a green truck that had stopped on the bridge just short of the edge and the upended tires of a tractor trailer in the rubble.

The collapse created an earthquake-like scene of destruction that left slabs of grey concrete and twisted strands of iron blanketing railroad tracks, buildings and a river bed filled with weeds and marsh grass.

One witness said simply: "I saw death."

There was initial confusion over the exact death toll and officials are still searching for people in the rubble after the incident at noon, local time.

Amalia Tedeschi, a firefighter, told RAI state TV that some 20 vehicles, including cars and trucks, had been involved in the collapse of a stretch of bridge some 80 metres in length.

The national police noted that there was a "violent cloudburst" at about the time the section fell, and some witnesses said the bridge was struck by lightning just before it collapsed, The New York Times reported.

Firefighters told the Associated Press they were worried about gas lines exploding in the area from the collapse.

One eyewitness, who gave his name as Andrea Rescigno and said he was in his car at the time of the bridge collapse, said in a phone interview with Genoa TV station Primocanale that he saw "cars and trucks plunging into the void".

"I saw death," Rescigno said. "My wife screamed at me to stop. If not for that we'd be dead now."

One “I am a miracle,” another witness, Davide Capello, told the Turin newspaper La Stampa. He said he was driving to the city when “I heard a noise first and everything collapsed.” His car fell and came to rest wedged between massive pieces of debris, yet somehow he was unhurt.

The bridge collapse occurred over a heavily populated area.

“I heard a loud noise, a sort of roar — strange, to be honest,” Marco Alloisio told the online news portal IVG.it. “I looked out of the window of my home and saw the bridge collapse all of a sudden, with all the vehicles traveling on it.”

He added: “I thought I was hallucinating or something similar, but that wasn’t the case. It’s an image that I will never forget.”

Bridge maintenance insufficient

Giovanni Toti, the governor of the northwestern Liguria region around Genoa, said maintenance work carried out on the bridge had proved insufficient.

"It's not acceptable that such an important bridge... was not built to avoid this kind of collapse," Italian Deputy Transport Minister Edoardo Rixi said on SkyNews24, speaking from Genoa. "The central section came down."

The bridge had been built in the 1960s and had been restructured two years ago, according to the Reuters news agency, which also said that work was ongoing at the time of the collapse to "shore up the foundation".

Italy, like many developed countries, has issues with aging infrastructure, but it was not clear what caused the bridge to give way. ANSA said authorities suspected that a structural weakness had caused the collapse.

Transport Minister Danilo Toninelli called the disaster an "enormous tragedy" and said Italy would need to perform checkups on highway bridges built between the 1950s and 1970s. He said that "maintenance was not done, and these facts go to show it".

"These kind of tragedies cannot and should not happen in a civil country," Toninelli said. "Those who will be considered responsible will need to pay to the last cent."

Italy's new government - a coalition of two populist parties that took power recently - had not placed an emphasis on road and highway spending in its platform.

"The tragic facts of Genoa remind us that the public investments of which we have absolute need are under everyone's eyes," said Claudio Borgi, the head of economic policy for Italy's ruling League party.

"Bridges, roads, aqueducts. Years of potential maintenance postponed and shelved 'because there's no money.' Italians' safety comes first."

Italian Premier Giuseppe Conte called the collapse "an immense tragedy ... inconceivable in a modern system like ours, a modern country."

Prosecutors said they were opening an investigation but had not identified any targets, AP reported.

Heavier traffic than usual

The disaster occurred on a highway that connects Italy to France, and northern cities like Milan to the beaches of Liguria. It came on the eve of a major Italian summer holiday on Wednesday called Ferragosto, which means traffic was heavier than usual as Italians travelled to beaches or mountains.

The Morandi Bridge is a main thoroughfare connecting the A10 highway that goes toward France and the A7 highway that continues north toward Milan.

The collapse of the bridge comes eight days after another major accident on an Italian highway, one near the northern city of Bologna.

In that case, a tanker truck carrying a highly flammable gas exploded after rear-ending a stopped truck on the road and getting hit from behind itself. The accident killed one person, injured dozens and blew apart a section of a raised eight-lane highway.

Agencies