Confessed mass killer Anders Behring Breivik's original plans for a terror attack was to bomb Oslo's government district, the Labor Party's office and a third target, possibly the royal palace, he told a court Thursday.

"There would be three car bombs, followed by a firearm-based action," Breivik said, on the fourth day of his trial.

Breivik has confessed to setting off a car bomb outside the government headquarters on July 22, and then opening fire at a Labor Party youth camp outside the capital. A total of 77 people were killed.

Questioned by prosecutors, Breivik said his original plan was to build three bombs. One would be placed at the government district and the second at the office of the governing Labor Party.

He had several options for the third target.

"I settled on the palace in a setting where the royal family wouldn't be hurt," he said. "Most nationalists and cultural conservatives are supporters of the monarchy, including myself."

The anti-Muslim militant said the three bombs would be followed by several shooting massacres, if he survived. He decided against multiple bombs because building one was "much more difficult than I thought."