At least 21 people including several children have died after a bus crashed off a flyover near Venice and burst into flames, officials say.

The electric bus broke through a barrier and plunged almost 15m (50ft) near railway tracks in Mestre, which is connected to Venice by a bridge.

Five Ukrainians, one German and the Italian driver were among the dead, city prefect Michele Di Bari said.

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

"An apocalyptic scene, there are no words," he said on social media.

CCTV footage of the flyover from Tuesday night showed the vehicle driving past another bus, before toppling off the carriageway.

One rescuer spoke of a "tragedy of young people, if not very young people, except for a few adults".

Three children including a baby were among the dead, emergency services said. Interior Minister Matteo Piantedosi said that the toll could rise.

Fifteen people are known to have been injured, five of them seriously. Venice officials said they included Ukrainians, Austrians, Spaniards and other foreign tourists.

Among the injured were two 16-year-olds and two younger children, the local governor said.

Two German brothers, aged seven and 13, were being treated for broken bones in hospital in nearby Treviso. Their parents were killed in the accident and the boys were being given counselling.

The bus, carrying 39 people, crashed at around 19:45 (17:45 GMT) on Tuesday. It had apparently been rented by a local company to pick up tourists from the historic centre of Venice to a campsite in the nearby Marghera district.

The bus company emphasised that the 13-tonne vehicle was electric, discounting earlier reports that it also ran on methane gas. Fire brigade commander Mauro Longo told Il Gazzettino website that the bus's batteries caught fire and made the task of clearing the bus a complex operation.

Witnesses said they could hear people screaming but the flames were too high to intervene.

A 27-year-old Gambian worker and a colleague were among the first people to reach the scene. He told how he had pulled three or four people from the bus, including a young girl.

One man called Leonardo said he heard the sound of strong braking before the sound of the crash, and he rushed to find out what had happened. "I wanted to help," he told La Presse website, "but I was prevented by a friend of mine and a policewoman because the bus was still in flames and in danger of blowing up."

What is unclear is why the bus left the flyover on a downhill stretch of the road and careered through a guard rail and metal barrier. Police are looking at video from security cameras near the crash site.

The 40-year-old driver, Alberto Rizzotto, had worked for the bus company for seven years and there was no indication on the road that he had tried to brake before the crash.

In his last Facebook post, he said he was running a "shuttle to Venice". One theory is that he may have become ill moments before the accident before losing control of the bus.

Massimo Fiorese from La Linea bus company, said the vehicle was less than a year old and the driver highly experienced.

"There's a video of the bus just before it falls," he told the Ansa news agency. "The vehicle arrives, slows down and brakes. It's almost at a standstill when it crashes through the guard-rail. I think the driver must have fallen ill, because otherwise I can't explain it."

Firefighters eventually removed the wrecked bus from the scene early on Wednesday.

A reception point staffed by psychologists and psychiatrists has been set up at a nearby hospital to provide support for the victims' families.

Italy's Prime Minister Giorgia Meloni said the country's thoughts were with the victims and their family and friends.