Norway is boosting security because of an imminent threat from people linked to Islamist militants in Syria, the Norwegian intelligence chief says.
The head of the PST security service, Benedicte Bjoernland, said it had "reliable information" about plans for some kind of attack "within days".
She called the threat "non-specific" but "credible".
Norway suffered a twin terror attack in 2011 when far-right extremist Anders Behring Breivik killed 77 people.
Norwegian police are stepping up their presence at airports, stations and border crossings because of the new threat.
A PST assessment last month said about 50 people had travelled to Syria from Norway as foreign fighters, and half of them had now returned to Norway, the Associated Press news agency reports.
Ms Bjoernland did not name any possible targets when she announced the alert, at a news conference with Justice and Security Minister Anders Anundsen and police chief Vidar Refvik.
The PST "recently received information that a group of extremists from Syria may be planning a terrorist attack in Norway", she said.
A Frenchman who travelled to Syria to join Islamists last year has been accused of murdering four people in a gun attack on the Brussels Jewish museum in May. Mehdi Nemmouche, 29, is now facing extradition from France to Belgium.
Other European governments, including the UK and France, have warned of the risk that "home-grown" Islamist volunteers may return from the Syrian conflict and carry out attacks.