Norwegian mass murderer Anders Behring Breivik said on Thursday he felt he had become "stranger and stranger" and more radical in his right-wing views in jail and blamed it on near-isolation since he massacred 77 people in 2011.

He expressed no remorse, however, for the massacre during a court hearing at which the state is appealing against a lower court ruling in 2015 that the tough conditions violate Breivik's human rights.

Breivik, who gave a Nazi-style salute at the start of the week-long court hearing on Tuesday, traced his feelings to a lack of critical feedback about his ideas while in jail.

"The last five years I've been completely isolated, not corrected a single time. I've sat in a cell 23 hours a day for almost 6 years ... I've become stranger and stranger as a direct consequence of this," he said.

"I've become a lot more radical while I've been jailed," he said, agreeing with allegations by Norway's attorney general in the court on Thursday. He added that he was "shocked by many of the things I have written".

Despite the Nazi salute, Breivik said that his underlying commitment was now to democracy and peaceful means.

On July 22, 2011, Breivik killed eight people with a car bomb outside the prime minister's office in Oslo and then shot 69 others on an island near the capital, many of them teenagers attending a youth camp of Norway's then-ruling Labour Party.

Attorney general Fredrik Sejersted told the court on Thursday that Breivik had to be kept away from other prisoners because he was still dangerous and wanted to spread a neo-Nazi ideology from inside the jail.

He is compensated with a three-room cell, including a personal gym, television, newspapers and playstation.

Breivik's lawyer, Oeystein Storrvik, said his client's treatment, including strip searches and use of handcuffs, violates a ban on "inhuman and degrading treatment" under the European Convention on Human Rights.

Breivik was more subdued than in the previous court appearance in 2015 when he joked about suffering ready-made meals and cold coffee. He is serving Norway's longest jail term, 21 years, which can be extended.