Oslo (AFP) - Norway is stepping up security amid intelligence reports of a possible imminent "terrorist attack" by militants who have fought in Syria, the country's security officials said Thursday.
The move comes as concerns mount in Europe about the growing threat posed by jihadists returning from war-torn Syria.
"We recently received information that a group of extremists from Syria may be planning a terrorist attack in Norway," said Benedicte Bjoernland, the chief of PST, the country's domestic intelligence service.
The threat is "non-specific" but "credible", said Bjoernland. Neither the timing of the attack, nor the identity of the militants are known, she added.
In a separate statement, Norwegian police said that the information received pointed to a possible attack "in Europe", with "Norway being specifically mentioned".
The authorities have ordered an increased armed police presence in stations and airports, recalled officers from leave and increased border controls.
The royal palace, parliament, Oslo city hall and other buildings usually open to the public have been closed to visitors.
"There is a specific threat against Norway and several measures have been implemented to face this threat," Justice Minister Anders Anundsen said, urging the population to be vigilant without stigmatising any group.
Prime Minister Erna Solberg postponed her summer holiday, but told local television that "it is important to live our lives and not be scared".
Oslo police were heavily criticised for their slow reaction to the massacre by right-wing extremist Anders Behring Breivik, who killed 77 people on July 22, 2011, but claim to have addressed the problem.
- Soaring jihadi numbers -
In its annual evaluation presented earlier this year, the PST said the threat level against Norway had increased because of the conflict in Syria.
The intelligence services said some 50 individuals with links to Norway had fought or were fighting in Syria since the conflict started, and about half of them have returned.
The Nordic country has one of the highest rates per capital of nationals who have travelled to fight in the Syrian conflict.
In May, the PST arrested three people suspected of trying to join the jihadi Islamic State of Iraq and the Levant (ISIL), now called simply the Islamic State (IS).
In a report from last December, the London-based International Centre for the Study of Radicalisation (ICSR) said the number of foreign fighters in Syria had almost doubled since last year to up to 11,000 from 74 countries.
"Among Western Europeans, the number has more than tripled from 600 to 1,900 now," the ICSR said.
The potential threat to security was underscored in May with the attack on the Jewish Museum in Brussels, where four people died.
The chief suspect, French-Algerian Mehdi Nemmouche, who spent more than a year in Syria, is thought to have joined some of the most radical and violent jihadist groups.
According to Cato Hemmingby, a researcher at the Norwegian Police University, the rare move to make the threat public could be an attempt to dissuade terrorists from staging an attack.
Only two weeks ago, US Attorney General Eric Holder met his Norwegian counterpart Anundsen in Oslo and called for cooperation with Europe to stem the "grave threat" of extremists travelling to Syria.