An overnight explosion at the US embassy in Oslo may have been an act of terrorism, Norway's police have said.

The embassy in the Norwegian capital sustained minor damage after the blast in the early hours of Sunday - but no-one was injured.

"One of the hypotheses is that it is an act of terrorism, but we are not completely locked into it," Frode Larsen, the head of police joint investigation and intelligence unit, told Norway's public broadcaster NRK.

Norwegian authorities say they are in contact with US diplomats, and an investigation into the incident is now under way. US diplomats have not commented.

Speaking to NRK later on Sunday, Larsen stressed that "we have to be open to the possibility that there may be other causes behind what has happened".

In an earlier statement, Norway's police said that "large resources" were sent to the area around the US embassy at about 01:00 local time on Sunday (00:00 GMT). 

"The police are in dialogue with the embassy and no injuries have been reported," the statement added.

Michael Dellemyr, who is leading the police response, told NRK that the explosion was at the public entrance to the building.

He said police officers had conducted searches in the area around the embassy in the Morgedalsvegen district of Oslo, about 7km (4 miles) outside the city centre.

He said the police had also issued an appeal for information from anyone who may have seen or heard anything about the incident.

Photos posted on social media later showed shattered glass in the snow outside the entrance to the consular section of the building, cracks in a glass door as well as dark marks on a tiled floor.

Norwegian authorities described the incident as "unacceptable", with Foreign Minister Espen Barth Eide stressing that "the security of diplomatic missions is very important to us".

He added that he had contacted US embassy chargé d'affaires Eric Meyer regarding Sunday's incident.