Police in Norway have released pictures of a suspect linked to an explosion outside the US embassy in Norway's capital, Oslo, on Sunday.

Two heavily pixelated images taken from surveillance footage show a person with their face concealed, dressed in dark clothing and carrying a backpack.

Police said in a statement on Monday that their investigation had so far showed the explosion was caused by an improvised device that was placed at the building's entrance. The blast caused minor damage and no injuries were reported.

Police previously said it was possible the explosion was an act of terrorism. They said in their latest update that they had no developments on the attacker's motive.

As well as the images, investigators were also examining a video published on Google Maps around the time of the incident, police said.

The now-deleted video featured Iran's Supreme Leader, Ayatollah Ali Khamenei, local media reported. He was assassinated last month during joint US-Israeli strikes on Tehran.

Police said the investigation into the embassy blast was being treated as high priority and that dogs, drones and helicopters had been used to carry out investigations at the scene of the explosion.

They appealed to anyone with information about the suspect, as well as anyone who noticed anything unusual in the area between midnight and 02:00 local time (01:00 GMT) on Sunday, to contact them.

Emergency services were dispatched to the embassy, in the western Morgedalsvegen district of Oslo, at about 01:00 on Sunday.

Following the incident, photos posted on social media showed shattered glass in the snow outside the entrance to the embassy's consular section, cracks in a glass door and 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".

A US State Department spokesperson said it too was investigating the incident.