Norway's intelligence service said Thursday it has received information about an imminent "concrete threat" against Norway from people with links to Islamic fighters in Syria.
Benedicte Bjoernland, the head of Norwegian security service PST, said the agency has received "reliable information" from a foreign partner about some kind of attack "within days." She declined to identify the source.
"It was unspecific about what the target might be," Bjoernland said, adding PST has no information about how or when such an attack would take place.
According to PST's assessment last month, about 50 people have traveled to Syria from Norway as foreign fighters, half of whom have now returned to Norway.
The domestic intelligence agency could not exclude that people involved with the threat already were in Norway.
Terrorism expects told The Nordic Page, a Norwegian English-language news site, that the state-owned energy giant Statoil could be a possible target.
Bjoernland told a news conference authorities hoped a terror act could be averted by going public with the information.
National Police chief Vidar Refvik said law enforcement would be more visible at border crossings, airports and train stations but reiterated the threat was vague about a target.
The greatest terror threat in Norway comes from Islamic extremists in and around Oslo, PST said in an April assessment.
It was unclear whether Thursday's case was linked to the May arrest of three Norwegian citizens with alleged links to an Al Qaeda splinter group on preliminary charges of supporting or participating in a terror organization.
Earlier this month, the United States put Norwegian citizen Anders Cameroon Ostensvig Dale on a terrorism blacklist.
One of the gunmen in the 2013 shopping mall assault in Nairobi, Kenya, was a Norwegian citizen.
Norway is still recovering from the 2011 attack by far-right fanatic Anders Behring Breivik, who killed 69 people in a shooting spree at a political youth camp, and eight others in a bombing of government headquarters.
The Associated Press contributed to this report.