Norwegian police are investigating an explosion outside the US embassy in the capital, Oslo, officials say.

Police received reports of a "loud bang" or explosion about 1am (11am AEDT) on Sunday, Oslo police said in a news release. No injuries were reported.

The explosion was caused by some sort of incendiary device, Oslo police representative Frode Larsen said during a news conference on Sunday.

Investigators believe the embassy was the target and are searching for the perpetrators and their motive.

Local media reported minor damage to an entrance of the embassy, and people nearby said 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 has not 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 Justice and Security Minister Astri Aas-Hansen.

"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 US embassy in Oslo referred media queries to the US State Department, which did not immediately return a request for comment. Nor did Oslo police.

Other details were not available.

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