Militant Islamists with fighting experience in Syria may be planning an attack in Norway in the coming days, police said on Thursday, as they deployed armed units at borders, airports and railway stations.

A small group of Norway-based militants who have gained combat experience in conflicts around the globe have become the biggest threat to the Nordic nation and up to 50 have travelled to Syria in recent years, police said.

"We have information indicating that a terrorist action against Norway is planned to be carried out shortly, probably within days," Benedicte Bjoernland, the director of the Police Security Service, the police's intelligence unit, said.

She said police had no information about the target or the nature of the planned attack so armed police, an unusual sight in Norway, would be deployed in places considered high risk.

Prime Minister Erna Solberg, who was informed of the threat late on Wednesday, delayed her holiday plans and would stay in Oslo while police mobilised both uniformed and undercover units, summoning many off-duty officers to work.

"It's most likely that there are foreign fighters involved, in which case we are talking about a small group, considering our knowledge of groups in Syria," Atle Mesoey, a security researcher at Norwegian University of Life Sciences, said.

"If police come out into the public with such a threat, it is highly credible, clearly," he said.

Neighbouring Sweden and Denmark said they were not raising their own threat assessment but highlighted the broader and growing risk from Islamist militants.

Denmark said it estimated more than 100 of its nationals have left for Syria. Sweden has said its biggest security threat comes from around 200 Islamists who could become involved in militant attacks, including young people radicalised after fighting in Syria.

Norway's biggest peacetime attack came three years ago when Anders Behring Breivik killed 77 people, mostly teenagers, at a youth camp, in a bombing and gun attack. Breivik said the attack was a fight against Muslim immigration. - Reuters