Norway's intelligence service says it has been warned of an imminent "concrete threat" against the nation from people with links to fighters in Syria.

Benedicte Bjoernland, the head of Norwegian security service PST, said on Thursday the agency had received "reliable information" from a foreign partner about some kind of attack "within days".

Bjoernland said there was no indication of a specific target.

She said the information involved people who have been fighters in Syria.

According to PST's assessment last month, about 50 people have travelled to Syria from Norway as foreign fighters, half of whom have returned to Norway.

Bjoernland declined to give further details.

The domestic intelligence agency could not exclude that people involved with the threat already were in Norway, adding it had called staff back from their summer breaks to further investigate the information.

Norway is still recovering from the 2011 attack by far-right fanatic Anders Behring Breivik, who killed 69 people in a shooting spree at a political youth camp, and eight others in a bombing of government headquarters.