The father of Anders Behring Breivik, who massacred 77 people in 2011, says his son's fascist, anti-immigrant views seem to be getting ever more extreme in jail.

Anders Behring Breivik detonated a bomb in central Oslo on July 22, 2011, which killed eight people.

He then took a boat to Utoya island, north-west of Oslo, where he shot dead 69 more people, most of them teenagers, who were holding a summer camp for the ruling Labour Party.

He was sentenced to 21 years in jail, the maximum in Norway though the term can be extended if there is deemed to be a risk he would commit new crimes.

Anders' father, Jens Breivik, told Reuters news agency he feared his son could influence other extremists from jail with his radical right-wing anti-immigrant ideology.

"I believe he is getting more and more extreme, maybe he will be more dangerous," Mr Breivik said.

Mr Breivik said his 35-year-old son sent him a rambling letter in which he refused his request for a first meeting since the murders.

The letter to his father demanded that he should declare himself a fascist as a pre-condition for meeting, as well as denounced Anders' grandparents for failing to side with Hitler's Nazis who occupied Norway during World War II.

Earlier this month, Anders wrote a separate letter in which he indicated he was renouncing violence and was willing to apologise, but set up a string of conditions including that he be allowed to set up a fascist party.

Breivik questions whether parenting influenced killings

Mr Breivik, who was launching his book My Fault? A Father's Story, said the discovery that his son was the killer exceeded a parent's worst nightmare.

In the book, he agonises over whether better parenting could have prevented the killings, saying he regretted once saying he wished his son dead.

"I was very upset, furious at him for what he had done," Mr Breivik said.

"I regretted saying that."

Since the massacre, Mr Breivik said he had been tormented by the question over why Anders committed the killings.

"I still think that if I had been a better father he might have been a better person today," he said.

Mr Breivik only had intermittent contact with his son after losing a legal battle for custody following a divorce from Anders' mother when he was four.

"I have often felt guilt and responsibility for what he did," Mr Breivik said.

Reuters