By Nicola Ruotolo, Gianluca Mezzofiore and Judith Vonberg, CNN

(CNN) -- At least 20 people died and 13 people were injured after a section of a highway bridge in northern Italy partially collapsed Tuesday, according to Angelo Borrelli, head of the Italian Civil Protection Agency.

Speaking to journalists, Borrelli said that around 30 vehicles and several heavy-duty trucks were on the affected section of the Morandi Bridge, which lies to the west of the port city of Genoa, when it gave way.

The number of casualties is expected to grow as the rubble is removed, Borrelli said, however it is unlikely that anyone was underneath the bridge at the time of the collapse, he added.

Italian news agency ANSA reported that several crushed vehicles are under the rubble with dead people inside, citing unnamed sources who did not provide a specific number of casualties. Some trucks ended up in the Polcevera river, Carabinieri police sources told ANSA.

In a separate report, citing sources in the fire and rescue services, ANSA said that "tens of people" were among the victims and that two people had been pulled out of vehicles alive.

Luca Cari, spokesman for the fire service, told Italian news agency Rai that rescuers were searching for people underneath the rubble as if this were an earthquake.

According to Rai, Italian Prime Minister Giuseppe Conte will travel to Genoa later Tuesday.

The A10 is a major highway for residents and tourists in Genoa, connecting the city with the nearby airport, and a key route along the Mediterranean, linking the Italian coast with French coastal cities to the west.

The incident occurred around noon (6 a.m. ET) Tuesday, Italian state police said, later posting a video on Twitter of the moment the bridge gave way.

Police said that a violent storm was the cause of the collapse. An automated report from the weather station at the nearby Genoa Cristoforo Colombo Airport around the time of the incident recorded thunderstorms with winds gusting to 57 kilometers per hour (35 miles per hour).

Shortly after the collapse, rescue vehicles were seen racing to the site. Several hours later, around 200 firefighters were still working at the scene, according to the Italian fire service.

Giorgio Larosa posted a video on Instagram showing rescuers working in heavy rain to free people from crushed vehicles in a grassy area below the bridge.

"The debris from the collapsed (bridge) fell 20 meters from my car," eyewitness Davide Ricci told local newspaper Il Secolo XIX. He said he saw the bridge give way as he drove south along the river road nearby. "The central pylon crumbled, then the rest came down," he said.

Later Tuesday, Giorgio Mascione posted a video on Twitter showing the large gap between the two remaining sections of the viaduct, and the piles of rubble beneath.

The bridge, also known as the Polcevera Viaduct, was designed by Italian civil engineer Riccardo Morandi and completed in 1968.

Maintenance works were underway to consolidate parts of the bridge, according to motorway operator Autostrade. A bridge crane had been installed to allow those works to be carried out, the operator said in a statement.

"The works and the status of the viaduct were subject to constant observation and supervision," the statement said. "The causes for the collapse will be the object of an in-depth analysis a soon as it will be possible to safely access the site."

Italy's Infrastructure Minister Danilo Toninelli tweeted, "I am following with the utmost apprehension what has happened to #Genoa and which looms as an immense tragedy. We are in close contact with Autostrade and we are going to the site with the deputy Minister Rixi. My total closeness at this time to the city."

In a tweet, Deputy Prime Minister Matteo Salvini thanked the rescue workers at the scene and said, "we are following the situation minute by minute."

This story has been corrected to reflect that the city of Genoa is located in northern Italy.