Three brothers have been arrested in Norway linked to an explosion outside the US embassy in the capital, Oslo, on Sunday.

The authorities believe the blast was caused by an improvised device that was placed at the building's entrance. It caused minor damage and no injuries were reported.

The brothers, all in their 20s, are Norwegian citizens with links to Iraq, Norway's police attorney Christian Hatlo told a press conference on Wednesday.

Among them is thought to be a suspect police had previously had released images of, broadcaster NRK reported.

"They are suspected of a terror bombing," Hatlo said. None of the brothers, who have not been named, were previously known to police.

The authorities are investigating several theories about the motivation for the attack, including whether a foreign state actor was involved.

"The most important thing is to get them [the suspects] questioned, then we'll see what happens," Hatlo said.

In addition to the images released earlier this week, investigators have been examining a video published on Google Maps around the time of the incident.

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

Dogs, drones and helicopters were used to carry out investigations as the scene of the explosion as police said the investigation into the embassy was being treated as high priority.

Emergency services were dispatched to the embassy on Morgedalsvegen Street in the Ullern district of Oslo at about 02:00 local time (01:00 GMT) 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 was investigating the incident too.