Norwegian mass killer Anders Behring Breivik wanted to capture and decapitate former Norwegian Prime Minister Gro Harlem Brundtland during his shooting massacre on Utoya island, he said.

Breivik told his trial he planned to film the beheading and post the video on the internet - as well as kill all 600 people who were at the Labour Party's youth camp on July 22.

But Brundtland had already left the area as the far-right fanatic began his shooting spree.

The revelation came as Breivik described how he became a deadly marksman by honing his gun skills by playing the violent Call Of Duty: Modern Warfare computer game. The mass killer even bought a special gun sight used on the game - and attached it to the hunting rifle he used for his deadly rampage on a Norwegian island.

Thomas Hylland Eriksen, a professor of social anthropology at the University of Oslo, said such computer games could put Breivik in a state of delusion.