Oslo - Six people died in a fire in a residential building in the Norwegian capital Oslo early Saturday, possibly as a result of the fire alarm being switched off and the only emergency exit being nailed shut, according to the police.

Thirty-three residents of the building were caught unawares by the fire as most were asleep, the police said.

As the fire service arrived three minutes after the alarm was raised at 3:42 am, many of the mostly young residents were sitting dazed on window sills - some in just boxer shorts and T-shirts - waiting to be rescued by ladder.

Some escaped onto the roof in sub-zero temperatures, while others jumped to the ground.

The front stairwell was ablaze and blocked as an escape route, while the rear exit was also unusable as the door had been nailed shut, according to residents.

The building consisted of many small, labyrinthine flats.

Some residents said that the fire alarm had been switched off for quite some time.

One woman sustained life-threatening injuries from inhaling smoke. Her condition was serious, but stable, the authorities said.

"It was like a labyrinth in there and very confusing with all the small flats and rooms," a spokesman for the fire service said after the fire broke out.

As all 49 residents of the building at number 31, Urtegata, in the Oslo district of Gronland were not yet accounted for, there could be more victims.

The cause of the fire was not immediately clear, but arson could not be ruled out.

The fire broke out on the first or second floor.

A fire service spokesman said the flames were fanned each time one of the residents opened their doors looking for an escape route. (dpa)