SKIEN (Reuters) - Norwegian mass murderer Anders Behring Breivik said on Thursday (Jan 12) 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 Norwegian 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 (Jan 10), said that he lacked critical feedback about his ideas while in jail and that he would benefit from contact with other inmates.

"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 in a subdued three-hour appearance in court, adding that he was "shocked by many of the things I have written".

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 said the court would have to decide whether Breivik, 37, truly regretted his neo-Nazi extremism or was merely pretending to sound contrite in order to persuade the court to give him more freedom in jail.

"He's saying what is rational for him to say in the circumstances," Mr Sejersted told Reuters.

On Wednesday (Jan 11), Mr Sejersted also argued that Breivik had become more radical in jail but drew the opposite conclusion to Breivik, arguing he should still be kept away from other prisoners because he was still dangerous and wanted to spread a neo-Nazi ideology.

Breivik, who only meets professionals such as guards and health personnel, is compensated with a three-room cell that includes a personal gym, television, newspapers and playstation.

He declined to answer when Mr Sejersted pressed him for a "yes"or "no" about whether he regretted the 2011 massacre.

Despite the Nazi salute and increasingly radical letters and writings in jail, Breivik told the court that his underlying commitment was now to peaceful means.

"I am not a militant any more, not since 2012," he said.

Breivik's lawyer, Mr Oeystein Storrvik, said his client's isolation violates a ban on "inhuman and degrading treatment"under the European Convention on Human Rights.

Breivik is serving Norway's longest possible jail term, 21 years, which can be extended.