The US embassy in Oslo was the target of an explosion overnight Saturday to Sunday which caused no casualties and "minor material damage", but could be due to a terrorist act, police announced, adding that they are searching for one or more perpetrators.

"One hypothesis is that this is a terrorist act, but we are not focusing solely on this lead. We must remain open to the possibility that there may be other causes," Frode Larsen, head of the joint police investigation and intelligence unit, told the public broadcaster NRK on the sidelines of a press conference.

The explosion hit the entrance to the consular section of the embassy at around 1:00 AM (00:00 GMT), according to police.

Images broadcast by the media showed shards of glass in the snow in front of the entrance, as well as cracks on a thick glass door and black marks on the ground at the foot of the door, probably caused by the explosion.

Investigators examined the scene overnight, while dogs, drones and helicopters were deployed to search for "one or more potential perpetrators", Oslo police said in a statement.

An incident described in a statement as "unacceptable" by Norwegian Foreign Minister Espen Barth Eide, who indicated that he had been in contact with Justice Minister Astri Aas-Hansen and the chargé d'affaires of the American embassy, Eric Meyer.

"The case is currently under investigation by the police and the Norwegian Public Security Service (PST)," the minister added.

The threat level in Norway has remained at three on a scale of five since November 2024, PST spokesman Martin Bernsen told AFP. He declined to say whether any threats had been made against US interests in Norway before the explosion.

- "Thick fog" -

Police commander Michael Dellemyr, interviewed by the private channel TV2, previously indicated that the police "had an idea of the cause" and that the explosion was certainly of human origin, and not accidental, while refusing to detail the type of damage.

A 16-year-old resident of the neighborhood, identified only by his first name, Edvard, told TV2 that he was watching television when he heard the explosion.

"My mother and I initially thought it was coming from our house, so we looked around, but then we saw flashing lights outside the window and a crowd of police officers," he said.

"There were police dogs, drones, police officers with automatic weapons and helicopters in the air," he added.

A group of three friends told TV2 that they were waiting for a taxi near the embassy. "We heard three 'bangs' that shook the ground," said Kristian Wendelborg Einung.

Once in their taxi, they drove back along the street next to the embassy, which they saw covered in smoke. "We arrived before the police. The layer of smoke was very strange. It was like a thick fog," he said.

Several hours after the explosion, police said the area around the building was "safe" for residents and passersby.

US embassies in the Middle East have been placed on high alert due to the offensive launched jointly with Israel against Iran on February 28. Several of them have been targeted by attacks from Tehran.