The 37-year-old right-wing extremist, who has been held in solitary confinement since being sentenced in 2012 to 21 years in prison, talked calmly in court before a three-judge panel.

He complained that he had been forced to strip naked in front of prison officials as frequently as five times a day.

Dressed in a black suit and tie, Breivik spoke coherently without emotion, saying he agreed with government lawyers who had warned that the self-proclaimed neo-Nazi had become more radicalised in prison.

"I have been damaged by the isolation ... (and) radicalisation has been a consequence of it," he said.

"I have not been a little hurt, I have been very damaged."

Breivik was speaking on the third day in a court case reviewing a government appeal against a ruling that his isolation in prison violates human rights.

Last year, Breivik sued the government, arguing that his solitary confinement, frequent strip searches and often being handcuffed in the early part of incarceration violated his human rights.

In a surprise decision, the Oslo District Court in April sided with his claim, finding that his isolation was inhuman and degrading and breached the European Convention on Human Rights, and ordered the government to pay his legal costs.

But it dismissed Breivik's claim that his right to respect for private and family life was violated by restrictions on contacts with other right-wing extremists, a decision that Breivik in turn is appealing against.

The government maintains that Breivik is dangerous and must remain isolated from inmates in the high-security prison in Skien, where the appeals case is also being heard.

Speaking before Breivik addressed the court, his lawyer Oystein Storrvik said the control over his communications with the outside world amounts to a "blatant breach of human rights".

Mr Storrvik argued that frequent censorship of his letters and long review periods mean Breivik cannot keep regular communication with people outside of the penal system.

Six days have been reserved for the hearings by the Borgarting Court of Appeals in the makeshift courtroom in the gym of the prison in Skien, 135 kilometres south west of the capital Oslo.

A ruling is expected in February.