OSLO, July 23, 2011 (AFP) - Norwegian Prime Minister Jens Stoltenberg on Saturday declared the twin shooting and bomb attacks which killed at least 91 people "a national tragedy."

"Never since the Second World War has our country been hit by a crime on this scale," Stoltenberg told a press conference.

At his side, Justice Minister Knut Storberget said however that "up to now, there is no reason to raise the threat level" of terror attacks in Norway.

"It is a nightmare," Stoltenberg said, speaking of the "fear, blood and death" experienced by young people at the summer camp where the worst of the carnage took place.

A gunman wearing a police uniform opened fire on young people attending the camp on Utoeya island, mowing down at least 84 of them.

"It is all the worse because Utoeya is a place I have visited every summer since 1974. I have known joy, commitment and safety there. Now the place has been through brutal violence and a paradise for youth has been turned into hell in a few hours."

Seven people were killed in a bomb attack which devastated government buildings in central Oslo hours before the carnage at Utoeya.

A 32-year-old Norwegian, named by media as Anders Behring Breivik, is being held as suspect in both attacks.