Fourteen people were killed and dozens injured when their bus plunged off a mountain road in Japan on its way to a ski resort, in the country’s worst such accident for 25 years.

Fourteen people were killed and dozens injured when their bus plunged off a mountain road in Japan on its way to a ski resort, in the country's worst such accident for 25 years.

Young skiers -- many in their teens or early 20s -- were asleep on the bus when it careened off the road before dawn in the resort town of Karuizawa.

"I heard people screaming 'Oh no, oh no' and then it hit the guard rail," a 22-year-old college student told broadcaster NHK. "Injured people were everywhere. I had to cover my eyes at the scene."

A 19-year-old survivor who was knocked unconscious in the crash told the broadcaster: "The bus was swaying right and left at massive speed. It was turning in an abnormal manner."

He said the next thing he had been aware of was the carnage after the smash.

"Someone woke me up and I saw a number of people being carried on stretchers. I feel very sad and I still can't imagine it was real."

Television footage showed the wrecked vehicle on its side in woodland, several metres (yards) from the crash barrier through which it had smashed as it drove off the highway.

There was no snow or ice on the roads when the accident happened and attention was focused on the driver of the overnight bus, which was travelling from Tokyo to the ski fields of central Japan.

The country has suffered a series of high-profile bus accidents caused by drivers falling asleep at the wheel.