TOKYO (AFP) - A Japanese bus operator whose vehicle plunged off a mountain road, killing 14 people and injuring dozens more, is suspected by officials of violating safety regulations, local media reported on Saturday (Jan 16).

Young skiers - many in their teens or early 20s - were asleep on the bus when it careened off the road before dawn in the ski resort town of Karuizawa on Friday, the country's worst such accident for 25 years.

Twelve students aged between 19 and 22 as well as the driver and a second driver were all killed, and 26 others injured.

Police are investigating bus operator ESP and Keyth Tour, a Tokyo travel agency which organised the ski package tour, but authorities have yet to announce the cause of the accident.

Transport Minister Keiichi Ishii on Saturday visited the site, from where the wrecked vehicle had already been removed, with the gnarled crash barrier still showing the impact of the collision.

Police suspect the driver lost control as he tried to make a sharp turn after the bus hit the barrier, according to broadcaster NHK.

But the Transport Ministry suspects the bus company may have violated safety regulations - by failing to give the driver written instructions on what route to take - according to the Yomiuri Shimbun and the Mainichi Shimbun newspapers.

One expert quoted by the Yomiuri also suggested the bus operator may be at fault.

"I suspect the bus operating company's negligent safety management could be behind the accident," said Dr Hajime Tozaki, professor of public transport policy at Waseda University.

The accident occurred only days after the authorities imposed penalties on the company for failing to give required medical check-ups to some drivers.

The operator did not immediately comment but the travel agency boss denied the company neglected safety standards and promised to help bereaved families.

But Mr Tomokazu Abe, whose daughter Marie, a 22-year-old student who was among the victims, hit out at the agency.

"They didn't contact us and I have called but no one answered," he told reporters. "I can't trust them. I can't feel their sincerity."

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

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