Flags flew at half-mast over Venice Wednesday after 21 people including a toddler were killed when a bus careered off an overpass and caught fire. 

"The bus flipped upside down. The impact was terrible because it fell from over 10 metres (32 feet)" landing next to railway tracks below, said Mauro Luongo, Venice's fire brigade commander.

The dead are thought to be mostly tourists returned from Venice's historic centre to a camping site on Tuesday evening. Fifteen others were injured in what Venice Mayor Luigi Brugnaro described as "an apocalyptic scene".

Firefighters said the bus was electric, despite the Italian interior minister earlier saying it ran on methane.

They spent hours extracting bodies from the charred remains of the bus, which was finally removed from the site in the early hours of Wednesday.

"Among the difficulties was the fact that the bus was electric so it had batteries. Unfortunately, they caught fire on impact," said Luongo. 

"That's why operations took a little longer to remove the vehicle."

Luca Zaia, the governor of the Venice region, confirmed the official death toll was 21, "including a one-year-old child and a teenager".

Five Ukrainians were among the dead identified so far, alongside a German, a Croatian, a Frenchman and the Italian driver, he said.

Five of the 15 injured are in a "very serious condition", and some were still being identified, he said.

Firefighters said the bus caught fire after careering off an overpass straddling a railway line and linking the mainland Mestre and Marghera districts of Venice in northern Italy.

Zaia said that flags on official buildings in the region would be put at half-mast because of this "tragedy of enormous proportions".

As to the cause, Zaia said: "The main hypothesis at the moment is that the bus driver... may have fallen ill."

Investigators are analysing surveillance cameras from the area as part of their investigation into what happened.

- Condolences -

Italian Prime Minister Giorgia Meloni expressed her "profound condolences". 

"I am in contact with Mayor Luigi Brugnaro and (Transport) Minister Matteo Salvini in order to follow the news of this tragedy," she said in a statement. 

According to Corriere della Sera newspaper, 19 people died at the scene, with the remaining two dying in hospital.

Francesco Moraglia, the Catholic Patriarch of Venice, was at the site where he blessed the dead, their bodies covered with white shrouds on which bouquets of red flowers had been placed.

French President Emmanuel Macron and European Union chief Ursula von der Leyen offered their condolences. 

German Foreign Minister Annalena Baerbock said she was "deeply saddened by the terrible bus tragedy... In this night of grief, my thoughts are with the victims, their families and friends."

In July 2018, a bus carrying a group of some 50 holidaymakers back to Naples fell off a viaduct near the city killing 40 people in all.    

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