Police in Norway investigate an explosion outside the US Embassy in Oslo

The explosion was caused by some sort of incendiary device, according to Frode Larsen, leader of the Oslo police joint unit for investigation and intelligence. Investigators believe the embassy was the target and are searching for the perpetrators and their motive.

Larsen said the embassy's entrance had damage, but no arrests have been made.

“We are early in the investigation, but we are working based on multiple hypotheses,” Larsen said in a statement. “Given the current security situation, it is natural to consider whether this was a targeted attack on the American Embassy. However, we have not committed to any single hypothesis.”

Local media reported that people nearby said the street was blanketed in thick smoke following the blast. Police are seeking to talk to witnesses.

A U.S. official said the incendiary device was inside a backpack and detonated outside the entrance to the Consular Affairs office at the embassy. The official spoke on condition of anonymity due to the sensitivity of the matter.

PST, the Norwegian police security service, called in additional personnel following the incident, but hasn't changed the country's terror threat level, according to communication adviser Martin Bernsen.

“This is an unacceptable incident that is being treated with the utmost seriousness," said Astri Aas-Hansen, Norway's minister of justice and public security.

"The police have stated that they are investigating the case with significant resources, and that nothing indicates the situation poses any danger to the public.”

The U.S. Embassy in Oslo referred media inquiries to the U.S. State Department, which said it was aware of the incident and was opening its own investigation.

“We thank the Norwegian authorities for supporting the investigation,” the statement said.

Dazio reported from Berlin and Lee reported from Washington.