At least 21 people, including two children and foreigners, were killed Tuesday and several others wounded when a bus running on methane plunged off a bridge in Venice and caught fire.

"A tragedy has struck our community this evening", mayor Luigi Brugnaro wrote on Facebook, describing the site of the crash as "an apocalyptic scene".

"The provisional toll is at least 21 fatalities and over 20 people hospitalised," said Luca Zaia, the governor of the Venice region, deploring a "tragedy of enormous proportions".

"Efforts are on to extract and identify the bodies," he said. "The victims and the wounded include people of several nationalities, not just Italians".

The bus was returning from Venice's historic centre to a camping site when the accident occurred around 7:30 pm (1730 GMT).

Firefighters said the bus caught fire after careering off a bridge straddling a railway line and linking the Mestre and Marghera districts of the northern Italian city.

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. 

Salvini said the cause of the accident could be the driver suddenly taking sick or becoming unwell. 

According to Il Corriere della Sera newspaper, the bus swerved off the bridge after smashing through the barrier and fell near railway tracks about 30 metres (100 feet) below.

The newspaper said it caught fire after hitting some power lines.

Interior Minister Matteo Piantedosi said, "The aggravating factor was methane and the fire this spread rapidly".

"I fear that the death toll will rise," he added.

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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