The US embassy in Oslo was hit by an explosion in the early hours of Sunday but no one was injured, police in the Norwegian capital said, adding the cause was not immediately known.

The blast occurred around 1:00 am local time (0000 GMT) and caused only "minor material damage" to one of the building's entrances, Oslo police said in a statement.

Investigators were examining the scene, while dogs, drones, and helicopters were involved in the search "for one or more potential perpetrators", it said.

"Police view such incidents in public spaces as very serious, and are investigating the case with substantial resources and high priority."

Police commander Michael Dellemyr told TV2 police would "not 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 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."

He said investigators were talking to witnesses, and TV2 reported that a bomb squad was at the scene.

Police said they were in contact with the embassy about the incident, and said several hours after the blast that the area around the building was considered "safe" for residents and passersby.

Police urged the public to report any tips or unusual observations from the area between midnight and 2:00 am (2300 GMT and 0100 GMT).

- Three 'bangs' -

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.

Residents near the embassy described hearing the explosion.

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 group of three friends meanwhile told TV2 they were waiting for a taxi near the embassy when the explosion happened.

"We felt three 'bangs' that made the ground shake," Kristian Wendelborg Einung said.

Once in their taxi, they drove past the scene and saw the street in front of the embassy covered in smoke.

"We arrived before the police. The blanket of smoke was very strange. It was like thick fog," he said.