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Police vehicles outside the U.S. embassy after a loud bang was reported at the site in Oslo, Norway, in the early hours of Sunday.Javad Parsa/Reuters

The U.S. embassy in Oslo was hit by a loud explosion early on Sunday, causing minor damage but no injuries, Norwegian police said, as the justice minister said a thorough investigation had been launched.

The blast at the embassy compound in western Oslo occurred at around 1 a.m. local time, sending thick smoke into the street by the entrance of the consular section, eyewitnesses said.

Police said they were searching for one or more potential perpetrators, with Justice Minister Astri Aas-Hansen saying they had deployed “considerable resources” to the investigation.

“This is an unacceptable incident that we are taking very seriously,” she told Norwegian press agency NTB.

The blast occurred at the entry to the consular section, Oslo police said, and witnesses said the entrance had been damaged.

“There was a very thick layer of smoke on the street,” said Sebastian Toerstad, 18, a high school student who drove past the embassy at the time of the explosion.

“There was some damage to the entrance,” Toerstad told Reuters.

Police said no further explosive devices had been found in the area.

“Investigations have been carried out at the scene with the aid of dogs, drones and a helicopter, searching for one or more potential perpetrators,” the Oslo police department said in a statement.

The U.S. embassy referred all inquiries to the U.S. State Department, when contacted by Reuters. The State Department did not immediately respond to a request for comment.