The US embassy in Oslo was hit by an explosion early on Sunday, causing minor damage but no injuries, Norwegian police said.
A loud blast was heard near the embassy compound in western Oslo at around 1 a.m., and smoke was seen rising from the compound, eyewitnesses told local media.
"We've determined that an explosion hit the American embassy," police spokesperson Mikael Dellemyr told public broadcaster NRK, adding that the blast occurred at the entry to the consular section.
No suspects have been identified at present, but police are searching for one or several perpetrators and are cooperating closely with the embassy, said Frode Larsen, head of the Oslo police investigation unit.
"One of our hypotheses is that this is terrorism, but we are also exploring other options," Larsen later told NRK.
It was not immediately clear what caused the blast or who was involved, the Oslo police department said in a separate statement.
"The police are in a dialog with the embassy, and there are no reports of any injured persons," the statement said.
Norwegian police: no increase in national threat
The Norwegian police have announced that while security around the embassy has been increased, the national threat level remains at the level it was before the explosion.
Police are currently searching for the person responsible for the explosion.
Norway's government was in contact with officials at the US embassy to convey that this was "an unacceptable act that we take very seriously," Foreign Minister Espen Barth Eide said in a statement.
The blast occurred at the entry to the consular section, Oslo police said, and witnesses said the entrance had been damaged.
Witnesses told local media outlet Verdens Gang (VG) that a canine unit, a drone, and a helicopter are among the resources being used at the scene, as well as the police's bomb squad and forensic teams.
“I was sitting watching TV when there was a bang and the whole house shook,” VG quoted a local as saying.
A second local added that the explosion had woken them up, and that they had seen "heavy smoke coming from the embassy.”