Defendant Philip Manshaus sits in a court room in Sandvika, Norway, as he waits for the verdict on Thursday. AP

A Norwegian court on Thursday sentenced a right-wing extremist to a minimum of 21 years in prison for a 2019 mosque attack near Oslo and murdering his step-sister in a racially-motivated act.

He was found guilty of trying to "kill as many Muslims as possible", though no one was seriously injured in the mosque attack.

"Philip Manshaus... is sentenced to 21 years of 'forvaring'," a court west of Oslo ruled, using the Norwegian term for a custodial sentence that can be extended indefinitely.

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A self-proclaimed neo-Nazi, 22-year-old Manshaus 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. One of them, a 65-year-old man, overpowered Manshaus.

Judge Annika Lindstroem reads out the sentence to Philip Manshaus (not in image) for terrorism in Oslo on Thursday. AFP

"Manshaus has said that his plan was not only to kill as many Muslims as possible but that wanted to destabilise society and accelerate the race war," the court said in its verdict.

The body of his 17-year-old step-sister, Johanne Zhangjia Ihle-Hansen, was later found in their home.

Adopted from China by his father's girlfriend, Ihle-Hansen was killed by four bullets in what prosecutors termed a "racist act".

Norway does not have a life sentence, but a custodial sentence can be extended without limit as long as the person is considered a threat to society. Manshaus can apply for an early release after 14 years.

The prosecution had called for the custodial sentence on the grounds that Manshaus would be "dangerous for a long time".

His defence lawyer Unni Fries had raised doubts about his mental health and recommended psychiatric care, against Manshaus' wishes.

Fries noted his "rather paranoid world view" and his belief that Europe was under threat from "Muslims, Jews, gender equality, homosexuals, the media and the authorities".

During the trial, three experts declared him legally sane and criminally responsible.