The explosion occurred around 1 am local time, the Oslo police department said in a statement.
Public broadcaster NRK quoted police incident commander Michael Dellemyr saying the blast hit the entrance of the embassy’s consular section.
“At around 1 am we received several reports of an explosion. We arrived shortly afterwards and confirmed that there had been an explosion that hit the US embassy,” he told NRK.
“There is minor damage,” he said.
“We are not going to comment on anything related to the type of damage, what it is that has exploded and similar details, beyond the fact that there has been an explosion” because “it is very early in the investigation”, he said.
He later told TV2 that police “have an idea of the cause”, adding: “It appears to us that this is an act carried out by someone.”
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He said investigators were talking to witnesses, and TV2 reported that a bomb squad was at the scene.
The police statement said investigators were in contact with the embassy about the incident and there was a huge police deployment on site.
Residents near the embassy said they heard a loud blast.
A 16-year-old identified only as Edvard told TV2 that he was watching television when he heard the blast.
“My mother and I first thought it came from our house so we looked around a little, but then we saw the flashing lights outside the window and a ton of police,” he said.
“There were police dogs and drones and police with automatic weapons and helicopters in the air,” he said.
A large area around the embassy was blocked off and police were stopping cars driving near the area, media outlets reported.
US embassies have been placed on high alert in the Middle East over American military operations in Iran and several have faced attacks as Tehran hits back at industrial and diplomatic targets.
But Dellemyr said there was no indication as yet that the incident at the embassy in Oslo was connected to the conflict.
“We’re not connecting it to the conflict. It’s far too early for that,” he told TV2.
