Oslo, Norway - Norwegian police said Friday they had arrested the mother of three brothers already detained on suspicion of bombing the US embassy in Oslo over the weekend, causing minor damage but no injuries.

Police arrested the three brothers on Wednesday, describing them as Norwegian citizens of Iraqi origin, all in their twenties.

One of the three has admitted to placing the improvised explosive device outside the entrance of the embassy's consular section in the early hours of March 8, his lawyer said.

The two others are suspected of being accomplices.

"Another person has been arrested and is suspected of being linked" to the attack, police said Friday in a statement.

"It is a woman in her 50s. She is the mother of the three brothers," the statement said.

While police qualified the blast as "powerful," it caused only minor material damage to the building.

US embassies have been placed on high alert in the Middle East due to American strikes on Iran.

Several have faced attacks as Tehran responds by targeting industrial and diplomatic facilities.