Norwegian police raised the country's security threat level following the discovery of an explosive device in central Oslo late on Saturday.
A 17-year old Russian citizen, who had lived in Norway since 2010 and had applied for asylum, was a suspect in the case and was in custody, police said on Sunday, after they set off a controlled explosion of a "bomb-like device" on Saturday night.
The risk of an attack in the coming months has been raised to "probable" from "possible", Benedicte Bjoernland, head of the police security service (PST), told a news conference.
"It is not yet clear whether the suspect was acting alone," she said.
The Oslo police bomb squad said the device was about 30 cm (1 ft) across and had appeared capable of causing only a limited amount of damage.
Police in the Nordic region have been on heightened alert after a truck ploughed into a crowd in the Swedish capital Stockholm on Friday, killing four and injuring 15 in what investigators said appeared to be a terror attack by an IS sympathiser.
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