Convicted Norwegian mass killer Anders Behring Breivik has threatened to go on a hunger strike unless he gets access to better video games, a sofa and a larger gym.

In a letter sent to The Associated Press and other media, Breivik writes that the hunger strike would continue until his demands are met or he dies.

Breivik's lawyer Tord Jordet confirmed Tuesday that the letter was authentic and said his client was waiting for a response from prison authorities on 12 demands before deciding whether to begin a hunger strike sometime during the next eight weeks.

Karl Hillesland, acting director of Skien Prison, one of the prisons where Breivik is serving his sentence, said "nobody is on hunger strike," adding that he would not comment on individual prisoners, in line with prison policy.

Breivik, a far-right fanatic, is serving a 21-year prison sentence for killing eight people in a bombing at the government headquarters and 69 more in a shooting spree at the left-wing Labor party's youth camp in 2011. His sentence can be extended for as long as he's considered dangerous. Legal experts say that likely means he will be locked up for life.

In the letter, which has a threatening tone, Breivik wants communication restrictions lifted and more frequent phone calls than the 20 minutes he says he is given per week. He also wants the available PlayStation 2 console replaced by a later version.