The authorities believe the blast was caused by an improvised device that was placed at the building's entrance, BBC reported.

The blast reportedly caused minor damage and injuries.

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

"They are suspected of a terror bombing," Hatlo said.

The arrested people were not previously known to the police.

The authorities are currently investigating the probable motives behind the blast.

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

The incident occurred at a time when security across US embassies and consulates have been heightened amid the Middle East crisis.

Multiple US diplomatic buildings in the Gulf countries, which host American troops, have been targeted in recent days by Iranian retaliatory strikes.

It is still not clear whether the blast in Oslo is linked to Middle East tensions.