"It is possible that some of these individuals now have the intention and the capacity to carry out violent attacks in Norway," the annual report presented by PST chief Benedicte Bjornland said.
The report stressed, however, that the appeal of the conflict among Norwegians was very limited. "Unless the conflict acquires a clearer ideological basis," it is unlikely to attract many Norwegian fighters to Donbas, the report said.
It warned about foreign fighters establishing right-wing extremist networks with radicals in Norway and other countries.
A ceasefire has been in place along the line of conflict in eastern Ukraine since February 2015. It was renewed last September and has generally been holding ever since, although the parties to the conflict have blamed each other for occasional shooting.