The U.S. embassy in Oslo was hit by a loud explosion early on Sunday, causing minor damage but no injuries, Norwegian police said.
The blast was heard from the embassy compound in western Oslo at around 1 a.m. (0000 GMT), and smoke was seen rising from the area, eyewitnesses said.
"There was a very thick layer of smoke on the street," said Sebastian Toerstad, 18, a high school student who drove past the embassy at the time of the incident.
"There was some damage to the entrance," Toerstad told Reuters.
Oslo police said the blast had occurred at the entry to the consular section.
Two police technicians wearing white overalls were later seen working at the site.
No further explosive devices had been found in the area, police said.
"Investigations have been carried out at the scene with the aid of dogs, drones and a helicopter, searching for one or more potential perpetrators," the Oslo police department said in a statement.
The embassy did not immediately respond to a request for comment outside of regular business hours.
The U.S. State Department did not immediately respond to a request for comment.

Russian strikes kill 20, exposing Ukraine's air-defence shortage
Clashes escalate at Sri Lanka prison to leave 25 dead, many more injured
Six killed in India's Mumbai as rains wreak havoc, disrupt travel
Wildfire in southern France forces 10,000 people from their homes
Folarin Balogun to play for US after FIFA suspends red-card ban