At least 21 people including two children have died after a bus crashed off a flyover near the Italian city of Venice and caught fire, officials say.

The bus broke through a barrier and plunged near railway tracks in the district of Mestre, which is connected to Venice by a bridge.

It had been rented to take tourists between Venice and a campsite in the nearby Marghera district, reports say.

Venice Mayor Luigi Brugnaro said a "huge tragedy" had taken place.

"An apocalyptic scene, there are no words," he said on X, formerly Twitter.

The incident happened at about 19:45 local time (17:45 GMT) as the bus was reportedly taking tourists back to the campsite.

Some reports say the bus was powered by methane gas and fell onto power lines and caught fire.

"The aggravating factor was methane and the fire this spread rapidly," said Interior Minister Matteo Piantedosi, warning that the death toll could rise.

Regional governor Luca Zaia said the dead and injured included people of "various nationalities, not just Italians".

The head of the local fire service said Ukrainian tourists were among those killed, while Italian news agency Ansa said Germans had also been among the passengers on board the bus.

The cause of the crash is not yet known and rescue workers are continuing to work at the scene.

At least 12 people were injured in the crash and up to five people were still missing, Ansa reported.

Italy's Prime Minister Giorgia Meloni said she was following developments and expressed condolences.

EU Commission president Ursula von der Leyen said she stood by Italian leaders in this "moment of profound pain".