An Islamist militant group with ties to Syria may be planning an attack in Norway, possibly within the coming days, Benedicte Bjoernland, the head of the Norwegian Police Security Service, PST, has said.

"The PST has received information that persons related to an extreme (Islamist) group in Syria may be planning to carry out a terrorist act in Norway," Ms Bjoernland told a news conference.

"They may be planning to carry out the attack within a short period of time, probably within a few days. But we do not know how, when and in what manner," she said.

Norwegian authorities said they were increasing the presence of police in stations and airports, recalling civil servants from their holidays and stepping up airline security.

The intelligence services said between 40 and 50 individuals with links to Norway had fought or were fighting in Syria.