OSLO:  The gunman behind Norway's massacres told court on Thursday he had planned an even bigger killing spree, with three car bombs and more shootings at targets across Oslo.

Anders Behring Breivik also said he had once spent a year playing video games, including role-playing online game "World of Warcraft", and a shooting game to prepare for what he believed would be a suicide mission.

The far-right extremist testified he had named his murder weapons after terms from Norse mythology, calling his rifle "Gungnir" after Odin's magical spear and his Glock pistol "Mjoelner" after Thor's hammer.

Breivik is on trial for July 22 twin attacks, when he killed eight people with a van-bomb targeting buildings housing the offices of Labour Prime Minister Jens Stoltenberg, who was not present at the time.

He then travelled to Utoeya island where, dressed as a police officer, for more than one hour he methodically shot at hundreds of people at a Labour Party youth summer camp, taking 69 lives, mostly teenagers.

On Thursday, on the fourth day of his trial, the 33-year-old said that originally "the plan was three car bombs followed by a shooting", describing the initial plan as a "very large operation."

He said he had considered placing a bomb near Labour Party headquarters.

For the third location, he considered parliament, Norway's Aftenposten newspaper and City Hall, before deciding on the royal palace, although he insisted he had planned to warn the royal family so they would not be hurt.

He said that, had he survived all three bombings, he would have used a motorcycle to drive first to a far-left squatter community, then to the Dagsavisen daily and finally to the headquarters of the Socialist Left Party, "executing as many people as possible" in each place.

"The plan was to not surrender before the whole plan had been carried out," he told the court. "It was a suicide mission where the probability of survival was equal to zero."

Breivik also answered questions from the prosecution about the year 2006, when he isolated himself to spend an average of 16 hours a day to play video games after returning to live with his mother at the age of 27.

"Some people dream about sailing around the world, some dream of playing golf. I dreamt of playing World of Warcraft," he told the court.

He insisted the game was a very social, not very violent strategy game, which was "pure entertainment (and) has nothing to do with July 22."

Instead, he said, it was a "hobby" and he decided to play it for a full year so as not to regret leaving a dream unfulfilled after his attacks.

"I felt it was right to do this to prepare myself mentally to sacrifice my life," he told the court.

Breivik also mentioned another game called "Call of Duty: Modern Warfare", which he said he had used as actual training for the shooting spree.

"It is a war simulator. It gives you an impression of how target systems work," he explained, adding he used it to practice "shooting other people".

Calm and more cooperative than Wednesday -- when he refused to answer questions about a network of far-right militants he claims to be part of called the Knights Templar -- Breivik smiled several times while discussing target shooting techniques.

When confronted about his smiles by prosecutor Svein Holden, he acknowledged the survivors and victims' families watching were probably reacting "in a natural way, with horror and disgust".

At the start of the day, the defendant refrained from making his habitual far-right salute -- touching his chest and extending his clenched right fist in front of him -- after objections from survivors and families.

Breivik, charged with "acts of terror", entered a plea of not guilty at the start of his trial, saying his actions were "cruel but necessary".

The gunman has told the court he wants to be executed or acquitted, deriding Norway's maximum 21-year prison sentence as "pathetic".

Breivik will only get prison if the court deems him sane -- something he is fighting for so as not to delegitimise his Islamophobic and anti-multicultural ideology.

While the sentence then would be the maximum 21 years, it could be extended indefinitely if he was still considered a threat to society.

If found insane he could be sentenced to closed psychiatric care, possibly for life.

Meanwhile, Oslo police said they had deported a German woman expressing support for Breivik, amid reports she claimed to be the gunman's lover. AGENCIES