At least 21 people, including two children and foreign tourists, 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."

A city hall official said the dead included Ukrainian tourists while Italian news agency ANSA said the fatalities included German and French citizens.

Three Ukrainians, a Croatian, German and French national were among the injured, a city official told AFP.

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.

An AFP photographer saw fire crews busy working at the charred remains of the vehicle that was overturned on its roof.

Firefighters were waiting for the vehicle's battery to cool before moving it to search for additional victims.

- 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.

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

According to the Corriere della Sera newspaper, the bus swerved off the bridge, 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 that the fire spread rapidly.

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

Francesso Moraglia, the Patriarch of Venice, was at the site where he blessed the dead -- 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".

Meanwhile, a spokesperson for the German foreign affairs department told AFP its embassy in Rome was working with Italian authorities to verify whether German nationals were among the victims.

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.