Norway has been warned of an imminent terrorist attack from an Islamist group with ties to Syria, according to the country's secret service chief Benedicte Bjoernland.

Norwegians have been given no specific advice other than to stay vigilant. Bjoernland said that according to "reliable information" from a foreign partner, the attack was planned within a short period of time, probably within a few days

"We don't know if it is within five or seven days, or how, when and in what manner," she said at a joint press conference with police commissioner Vidar Refvik and justice minister Anders Anundsen, who cancelled his holiday to be present.

"The PST [Police Security Service] has received information that persons related to an extreme (Islamist) group in Syria may be planning to carry out a terrorist act in Norway," Bjoernland said. "We don't know what goal, who is behind or what is the target."

On the 23 July, Norway commemorated three years since the Oslo and Utoya Island attacks that left 77 people dead. A car bomb killed eight people and injured around 209 more while Anders Behring Breivik's shooting spree on Utoya Island killed 69 people and injured a further 110.

The island was hosting a summer school for the AUF, the youth division of the ruling Norwegian Labour Party.