Oslo (AFP) - Norwegian mass murderer Anders Behring Breivik has said he is ready to go on hunger strike until death if he is not better treated in prison.

The right-wing extremist, who killed 77 people in a bomb and gun rampage in 2011, is serving a 21-year sentence that can be extended if he is still considered a danger to society.

News site Nettavisen on Tuesday reported receiving an "open letter" addressed to various media outlets and penal authorities in which the 36-year-old complains that the conditions of his detention have become even stricter since September 2.

"Unless the 02/09/15 escalation is reversed, I will eventually continue a hunger strike until death," Nettavisen quotes him as saying in the letter.

Breivik's lawyer was not available for comment on Wednesday.

Breivik, who has described his solitary confinement as a form of "torture", exhausted his legal options last year when police closed their preliminary investigation into his complaint.

Letters to the media are now his only means of protesting about his treatment in Skien prison in southeast Norway.

In January 2014, he wrote to AFP with a list of 12 demands that included an upgrade of his PlayStation 2 games console to a PlayStation 3.

He has repeatedly warned that he may go on a hunger strike over his treatment, but has yet to do so.

Norwegian authorities do not usually comment on conditions of detention.

Citing opposition to multiculturalism and what he called a "Muslim invasion", Breivik killed eight people in a bombing in Oslo on July 22, 2011 before shooting 69 others -- most of them teenagers -- who were at a Labour Party summer camp on the island of Utoya.