MONTEFORTE IRPINO, Italy — Thirty-nine people were killed and around 10 injured when a bus plunged off a viaduct in southern Italy in what Prime Minister Enrico Letta described on Monday as a huge tragedy.

Initial reports suggested that the coach was traveling at speed and had hit four or five cars before crashing over the roadside barriers on a stretch of road near Monteforte Irpino, east of Naples, on Sunday night. There were around 50 people on board, including many children.

It was one of the worst road accidents in Italy's history, and the second transport disaster to hit southern Europe in the space of five days. Seventy-nine people were killed when a high-speed train derailed in the Spanish pilgrimage town of Santiago de Compostela on Wednesday.

Letta, on an official visit to Athens, canceled a private visit to the historic Acropolis before meetings with his Greek counterpart Antonis Samaras.

"It is a very sad day for Italy, what happened last night. There are no words for it," he told reporters. "It is a huge tragedy."

A statement from motorway operator Autostrade per l'Italia said the coach appeared to have been traveling fast in the vicinity of slower-moving traffic, even though the lower speed had been clearly indicated.

"You would think that the barriers on the viaducts and bridges should prevent this type of accident but evidently it seems the impact was so strong that even the barrier gave way," said Alessio Barbarulo, head of the local fire brigade division that coordinated the rescue effort.

As rescue services worked, crushed wreckage from the coach lay on its side, while bodies covered in white sheets were lined up on the road before being taken away by ambulances.

Thirty-six people were killed immediately and another three died later in hospital, a police official said.

Infrastructure Minister Maurizio Lupi said it was too early to say what may have caused the accident but that there was no indication of technical problems with the bus, which had passed its annual inspection in March.

An official from the prosecutor's office in nearby Avellino said investigations would look at the possibility of manslaughter or causing an accident. The driver was killed in the crash.

All of the victims appear to have been Italians returning from an excursion to the town of Telese Terme, known for its hot sulfurous springs and a nearby lake popular as a fishing spot.

Local media said the stretch of road where the bus crashed had been the scene of repeated accidents.