OSLO (AFP) - Three people were killed when a man armed with a knife hijacked a bus in western Norway on Monday, according to police, who have arrested a suspect they say comes from South Sudan.

Police commissioner Ronny Iden told reporters that the suspect, a 31-year-old man, lived in Aardal, a small town near the scene of the attack.

The victims were two men in their 50s - the bus driver and a Swedish passenger - and a 19-year-old woman, police said, adding that they were the only ones on the bus besides the suspect.

The suspect also suffered knife wounds and was currently hospitalised in Bergen, Norway's second largest city, he added.

The motives for the attack were not immediately clear.

The alleged attacker was overpowered by firefighters who rushed to the scene of what they initially believed was a traffic accident, police said.

The long-distance bus was on the route between the mountainous Valdres region, a popular area for skiing, and the Norwegian capital Oslo.

The nearest police station was about 89 kilometres from the deadly attack and first officers arrived at the scene about an hour after the alert was given.

Nothing suggests that the alleged perpetrator and the victims knew each other, police said, adding that police criminal investigators were coming from Oslo to help with the inquiry.

A witness who was the first to arrive at the scene said he initially believed there had been an accident and rushed to help people inside the vehicle.

"The bus was on the side of the road, so we stopped our car and ran over," the witness told TV2 Nyhetskanalen, who identified himself only by his first name Leif.

He said he and another person tried to open the doors, but in vain.

"It was impossible to open the doors. Then we saw a dark-skinned person inside the bus. At first, we thought he was trying to get out but then saw he was moving around with a knife, and we realized that the situation was quite different," he said.

In 2003, on the same route between Valdres and Oslo, an Ethiopian killed the driver of a bus after murdering an asylum seeker in a local shelter.

He was sentenced to treatment in a psychiatric hospital.