Emergency services in Italy are continuing to search for survivors at the site of a bridge collapse in the port city of Genoa that killed at least 39 people and injured many others.

Key points: More than 30 cars and trucks believed to be involved in the collapse

More than 30 cars and trucks believed to be involved in the collapse Four people pulled alive from vehicles under the bridge

Four people pulled alive from vehicles under the bridge Italian Prime Minister Giuseppe Conte said collapse is "an immense tragedy"

The bridge, which is on a major highway, collapsed yesterday during a sudden, violent storm, sending vehicles plunging 45 metres into a heap of rubble below.

Italian authorities raised the death toll to 39, with Angelo Borrelli, the head of the civil protection department telling a news conference that some 1,000 rescuers have been working since after the collapse Tuesday to search for "any possible missing" persons.

He added that 15 people were injured in the bridge collapse.

Italian Prime Minister Giuseppe Conte called it "an immense tragedy … inconceivable in a modern system like ours, a modern country".

The head of the country's civil protection agency, Angelo Borrelli, said all of the victims appeared to have been in vehicles that plunged from the bridge.

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, Italian news agency ANSA reported.

Tons of twisted steel and concrete debris collapsed onto warehouses and other buildings. ( Reuters: Stefano Rellandini )

"We are still trying to extract survivors from the rubble," Genoa police official Alessandra Bucci said.

"We hope to find more people alive."

There was initial confusion over the exact death toll.

Earlier, Mr Borrelli said 30-35 cars and three heavy trucks were caught up as an 80-metre stretch of the bridge collapsed.

"We need to remove all of the rubble to ascertain that all of the people have been reached," Mr Borrelli said, adding that more than 280 rescue workers and sniffer dogs units were at work.

"Operations are ongoing to extract people imprisoned below parts of the bridge and twisted metal."

One man who was standing under the bridge in front of his truck when the span collapsed called it "a miracle" that he survived.

Sorry, this video has expired Authorities were still hoping to find survivors. (Photo: Vigili Del Fuoco via AP)

The middle-aged man, who did not give his name, said he was sent flying more than 10 metres into a wall, injuring his right shoulder and hip.

"I was in front of the truck and flew away, like everything else," he said, walking away from the site.

"Yes, I think it's a miracle. I don't know what to say. I'm out of words."

A truck is stopped right at the edge of the collapsed bridge. ( AP: Antonio Calanni )

Motorist Alessandro Megna told RAI state radio he had been in a traffic jam below the bridge and saw the collapse.

"Suddenly the bridge came down with everything it was carrying," he said.

"It was really an apocalyptic scene, I couldn't believe my eyes."

Mr Borrelli said authorities were still trying to figure out the reason for the collapse.

The highway operator said work to shore up the bridge's foundation was being carried out at the time of the collapse, adding that the bridge was constantly monitored.

Rescuers counted survivors among the bodies after the motorway collapse. ( AP: Luca Zennaro )

Firefighters said they were worried about gas lines exploding in the area from the collapse.

Photos published by ANSA on its website showed a huge gulf between two sections of the bridge.

Video captured the sound of a man screaming: "Oh God! Oh, God!"

Other images showed a green truck that had stopped just short of the gaping hole in the bridge.

Dozens of cars and trucks were caught up in the collapse. ( Reuters: Stefano Rellandini )

'Those responsible will have to pay'

As rescue efforts continued, Italy's transport minister Danilo Toninelli called for senior managers to resign at the company operating the bridge.

The Government will also look into stripping Autostrade per l'Italia, a unit of the Atlantia group, of the concession to manage the motorway that included the Morandi Bridge, and imposing financial penalties on the group, Mr Toninelli said.

"Autostrade per l'Italia was not able to fulfil its obligations under the contract regulating management of this infrastructure," he said on RAI 1 state television.

"I have given mandate to my ministry to start all proceedings to apply the agreement, that is to revoke the concession from these companies and seek significant sanctions which can reach up to 150 million euros based on the terms of the contract."

Rescuers work among the debris of the collapsed Morandi highway bridge. ( AP: Luca Zennaro/ANSA )

Atlantia and Autostrade per l'Italia could not immediately be reached for comment.

Earlier, Mr Toninelli, said the disaster showed the dilapidated state of Italy's infrastructure and a lack of maintenance, adding that "those responsible will have to pay".

"We are following minute-by-minute the situation of the bridge collapse in Genoa," Interior Minister Matteo Salvini said on Twitter.

The disaster happened 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, which means traffic was heavier than usual as Italians travelled to beaches or mountains.

Hundreds of rescue workers were searching for victims. ( AP: Luca Zennaro )

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

Inaugurated in 1967, it is 90 metres high and just over a kilometre long, with the longest section between supports measuring 200 metres.

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, leaving one person dead and dozens injured.

The Morandi bridge was built in the 1960s. ( AP: Luca Zennaro )

AP/Reuters