A Norwegian man who admitted to killing his step-sister before opening fire in a mosque near Oslo last year pleaded not guilty as his trial started on Thursday.

Wearing a dark suit, 22-year-old Philip Manshaus appeared before the court outside Oslo, making the "OK" sign with his hand, used by some to signify white supremacy, as he entered.

Manshaus stands accused of murder and committing an act of terror.

He was arrested on August 10, 2019 after opening fire in the Al-Noor mosque in the affluent Oslo suburb of Baerum while wearing a bullet-proof vest and a helmet with a camera strapped to it.

Just three worshippers were in the mosque at the time, and there were no serious injuries as a 65-year-old man overpowered Manshaus.

According to the charge sheet, Manshaus' aim was to "kill as many Muslims as possible".

The body of his 17-year-old step-sister was later found in their home.

Norwegian Philip Manshaus (right), with his lawyer (left) pleaded not guilty to opening fire at a mosque near Oslo last year
Norwegian Philip Manshaus (right), with his lawyer (left) pleaded not guilty to opening fire at a mosque near Oslo last year NTB Scanpix / Lise Åserud

Adopted from China by his father's girlfriend, Johanne Zhangjia Ihle-Hansen was killed by four bullets, police said.

Manshaus has admitted to the facts of the case but pleaded not guilty, claiming his actions came out of "necessity".

Norwegian media reported that Manshaus showed no remorse in the courtroom but rather expressed regret that he had "not been able to inflict more damage".

According to the prosecution, Manshaus had a racist motive and was inspired by the attacks in Christchurch in New Zealand in March 2019, when Brenton Tarrant killed 51 people in shootings at two mosques.

Tarrant in turn has said he was inspired by Norwegian Anders Behring Breivik, who in July 2011 killed 77 people in a truck bomb blast near government offices in Oslo and a shooting spree at a Labour Party youth camp on the island of Utoya.

The Manshaus trial is due to last until May 26.

He faces 21 years in prison if convicted, but the prosecutor's office has said they have not ruled asking for a custodial sentence, which would keep Manshaus behind bars until he is no longer deemed a danger to society.