OSLO- An assailant who shot dead at least 84 people at a youth camp of Norway's ruling party turned a "youth paradise into a hell", Norwegian Prime Minister Jens Stoltenberg said on Saturday.

"Many of those who lost their lives were persons I know. I know the young people and I know their parents," he told a news conference.

"And what hurts more is that this place where I have been every summer since 1979, and where I have experienced joy, commitment and security, has been hit by brutal violence -- a youth paradise has been transformed into a hell."    

"What happened at Utoeya is a national tragedy," he said of the small wooded holiday island that was hosting the annual camp for the Labour Party's youth wing. "Not since World War II has our country seen a greater crime."

The prime minister said he did not want to speculate on the motives of the attacks, but added: "Compared to other countries I wouldn't say we have a big problem with right-wing extremists in Norway. But we have had some groups, we have followed them before, and our police is aware that there are some right-wing groups".

"There have been some groups of that kind in Norway, but again I will not speculate. We will await the investigation from the police before we say anything about the case."

Police said at least 84 young people had been killed, as well as employees at government offices in Oslo, where a bomb killed seven people earlier on Friday.

"It's beyond comprehension and it's like a nightmare. A nightmare for the young who were killed, for their close ones; mothers, fathers and siblings who were brutally confronted with death. But also for the survivors and their kin," said Stoltenberg.  

"Each and everyone who was present at Uteoya is damaged for life. Young people have experienced things every person should be spared -- fear, blood and death.