"Asylum seekers linked to radical Islam are not a main concern to the PST in the short-term," intelligence service PST said in a statement.
"The increasing flow of asylum seekers in Norway could, first and foremost, have negative consequences on threats linked to far-right circles in Norway. This is because hostility to immigration is one of the main issues, and an important mobilising factor, for these circles," it said.
The Scandinavian country is still traumatised by its inability to prevent right-wing extremist Anders Behring Breivik's July 2011 twin attacks. He killed 77 people over his opposition to multiculturalism and what he called "the Muslim invasion."
Norway has never experienced a deadly Islamist attack on its soil. Some on social media have suggested that members of the Islamic State or other extremist Islamic groups may be slipping unnoticed into European countries amid the large influx of migrants, in order to carry out attacks one day.
"The threat linked to radical Islam comes primarily from people born or raised in Norway, and who have been radicalised here," PST said.
The statement added that the far-left could also pose a threat, and noted that the two sides could face off in violent clashes.
In the first eight months of the year, Norway registered more than 8,000 asylum seekers, of whom about a quarter are Syrian, with the numbers rising in recent weeks.
Immigration authorities expect up to 20,000 asylum applications for the full year, which would be a record.