OSLO (AP) — Police in Norway were investigating an explosion early Sunday outside the U.S. Embassy in Oslo, the capital of the Scandinavian country, officials said.

No injuries were reported. Police received reports of a “loud bang” or explosion around 1 a.m., Oslo police said in a news release.

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 that 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 that the street was blanketed in thick smoke following the blast. Police are seeking to talk to witnesses.

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 queries to the U.S. State Department, which didn't immediately return a request for comment. Nor did Oslo police. Other details weren't available.