Anders Behring Breivik, who killed 77 people in Norway last July, said today he had originally planned to set off three car bombs rather than one before going on a shooting rampage in Oslo.

"The plan was three car bombs followed by a shooting," the right-wing extremist told an Oslo court on the fourth day of his trial, describing the initial plan as a "very large operation."

Questioned by the prosecution, he said he had planned setting two one-tonne bombs, one in the government district and the other near the headquarters of the Labour Party.

He had also thought about a third bomb, a 500 kilo device, he said, but was undecided on the target.

He cited a tower block housing the Norwegian newspaper Aftenpost; the parliament building; Oslo city hall and the royal palace as possible targets.

Breivik told the court that, had he survived these initial attacks, he would have planned further gun attacks.

He intended to target the famous "Blitz" squat in Oslo; the Dagsavisen newspaper and the Norway's Socialist Left Party, whose offices were all close to each other.

In the end, he planted only the first bomb when he carried out his attacks on July 22, 2011.

But he followed that up with a shooting rampage on Utoeya island where hundreds of people were attending a Labour Party youth camp.

The shooting spree claimed 69 lives, mostly teens trapped on the small island surrounded by icy waters. It was the deadliest massacre ever committed by a lone gunman.