A bus crash killed 38 people and injured 18 after a bus plunged 25 metres off a viaduct in Italy.

Initial reports suggested the coach was travelling at speed and hit four or five cars before crashing over roadside barriers near Monteforte Irpino, east of Naples last night.

There were around 50 people on board, many of them children.

It was one of Italy's worst road accidents.

Prime Minister Enrico Letta, on an official trip to Athens, cancelled a private visit to the Acropolis.

He said: "It is a very sad day for Italy, what happened last night. There are no words for it, It is a huge tragedy."

A statement from motorway operator Autostrade per l'Italia said the coach appeared to have been travelling fast near slower-moving traffic, even though a lower speed limit had been clearly indicated.

Officials were examining onboard instruments and the guardrail on the A16 highway where the accident occurred to try to determine the cause of the crash.

As rescue services worked, the crushed wreckage of the coach lay on its side.

Bodies covered in white sheets were lined up on the road before being taken away by ambulances.

There was some initial confusion over the number of casualties but a police official said 38 people died in the accident.

A police official said 35 were killed immediately and another three died later in hospital.

Eighteen people, including a number who were travelling in cars struck by the coach, were injured.

Infrastructure Minister Maurizio Lupi said it was too early to say what may have caused the accident.

He said that there was no indication of technical problems with the bus, which had passed its annual inspection in March.

The driver was killed in the crash and an autopsy has been ordered on his body, officials said.