A Norwegian man suspected of killing his ethnic Chinese stepsister and then storming an Oslo mosque and opening fire said on the first day of his trial that it was an act of "emergency justice" and that he regretted not having caused more damage.

Philip Manshaus appeared at a court west of Norway's capital on Thursday (local time) and denied charges of murder and terror read to him by a prosecutor, Norwegian news agency NTB said.

He has acknowledged the facts but denies the accusation, saying he opposes non-Western immigration.

Broadcaster NRK said that during his testimony the accused spoke about his own racial views and criticised those who "blackmail national socialism".

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In court, the 22-year-old described how he killed his 17-year-old stepsister, Johanne Zhangjia Ihle-Hansen, by shooting her four times — three in the head and one in the chest — with a hunting rifle at their home in the Oslo suburb of Baerum.

Ms Ihle-Hansen was adopted from China as a two-year-old.

Shortly after that, Mr Manshaus said he drove to a nearby mosque where three men were preparing for Eid al-Adha celebrations.

He wore a helmet with a video camera attached and a bulletproof vest.

He was armed with a hunting rifle and a shotgun and fired four shots with the rifle at a glass door before he was overpowered by one of the men in the mosque at the time, Muhammad Rafiq.

During the scuffle, two more shots were fired but no one was hit. Mr Rafiq was slightly injured in the struggle.

In his testimony, Mr Manshaus said he wanted to kill Mr Rafiq.

"I did everything I could to carry out the attack," he told the court, adding that he was exhausted when police arrived.

Some 30 witnesses, including the men at the mosque and the father of the accused, are expected to give evidence.

If found guilty he could face up to 21 years in prison. The prosecution said it would consider a sentence where he would be sent to a secure mental facility for as long as he is considered a danger to others.

Norwegian media have reported that the accused was inspired by shootings in March 2019 in New Zealand, where a gunman targeted two mosques, killing 51 people, and in August 2019 in El Paso, Texas, where an assailant targeted Hispanics and left at least 22 dead.

His plans and reference to national socialism also recall those of Norwegian right-wing extremist Anders Behring Breivik, who in 2011 killed 77 people in a bombing and shooting rampage.

Breivik is serving a 21-year prison sentence for carrying out a terror attack.

Norway's domestic security agency PST said it had a "vague" tip about the accused a year before the shooting, but it was not enough to act on because they had no information about any "concrete plans" of attack.

AP