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.

 Young passengers

Chief Cabinet Secretary Yoshihide Suga told a news conference in Tokyo that as well as the 14 people who died, 27 others were hurt. He said no other vehicles were involved.

Police told AFP that the dead included both the 65-year-old driver and alternate driver, who was aged 57. 

The mass-selling Yomiuri Shimbun said the older man was at the wheel at the time of the accident.

The accident drew banner newspaper headlines and dominated television news bulletins throughout the day in a country where road safety standards are high.

The Yomiuri said it was the first bus accident since 1990 in which more than 10 people were killed.

Media said the passengers ranged in age from 18 to 32, and most were university students.

The travel agency that arranged the tour said that the bus was not on its originally-scheduled route.

Government spokesman Suga said the transport ministry had launched an investigation into the accident, sending inspectors to the site, while police separately plan to raid the bus operator and the Tokyo travel agency that organised the package tour.

Rules governing the working conditions for late- and long-distance highway bus drivers were tightened after an April 2012 accident that left seven people dead.

That bus hit a highway wall after its driver dozed off at the wheel. Many of its passengers were on their way to visit the Tokyo Disney Resort.

Japan has an extensive and well-maintained network of highways throughout its mountainous terrain.