Norway's prime minister has condemned the "cowardly and evil attacks" against his country after a bomb ripped through Oslo's government district, killing seven people.

In a news conference, Jens Stoltenberg addressed the attackers directly saying: "No one will bomb us to silence, no one will shoot us to silence, no one will ever scare us from being Norway."

Nine people were thought to have been killed in a separate attack at a youth Labour Party camp on an island outside the capital, when an armed man dressed as a police officer opened fire.