OSLO (AFP) - Norway has taken exceptional security measures after being informed of a possible imminent "terrorist attack" by militants who have fought in Syria, the country's intelligence chief said Thursday.

The move comes as concerns are mounting in Europe about the growing national security threat posed by jihadists returning from war-torn Syria.

The domestic intelligence service (PST) "recently received information that a group of extremists from Syria may be planning a terrorist attack in Norway," said PST chief Benedicte Bjoernland, adding it could be a question of days.

The threat is "non-specific" but "credible", said Bjoernland. Neither the eventual target, nor the identity of the militants, nor their location are known, she added.

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

In its annual evaluation presented at the start of the year, the PST said the threat level against Norway had increased because of the civil war in Syria.

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