* Attack may come within next few days
* Suspected militants fought in Syria
* Several hundred across the Nordics may have been to Syria (Adds police response, prime minister, numbers on potential threat, security analyst)
By Ole Petter Skonnord and Balazs Koranyi
OSLO, July 24 (Reuters) - 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.
A botched suicide bomb attack four years ago in Stockholm and the conviction in 2012 of three Swedes for plotting to kill people at a Danish newspaper after it published cartoons of the Prophet Mohammad in 2005 have shown the Nordic countries are not immune to attacks.
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.
NATO-member Norway has been working to clamp down on militant activity. In May it arrested three people suspected of aiding the Islamic State in Iraq and the Levant, an al Qaeda offshoot fighting in Iraq and Syria that now styles itself Islamic State. (Additional reporting by Alistair Scrutton in Stockholm and Sabina Zawadzki in Copenhagen; Editing by Alison Williams; Editing by Alistair Scrutton)