An investigation is under way after a blast in central Oslo as police confirmed a suspect has been arrested.

Norwegian media reported the suspect was a 13-year-old.

No injuries were reported at the site of the explosion, which was located near a university campus and 500 metres from the Israeli embassy.

Norwegian police later conducted a controlled detonation of a second explosive found on the site.

In an unusual move, authorities sent an emergency message to Oslo residents’ mobile phones, warning them about the blast.

The second device found appeared to be a “military-style” hand grenade, the police officer in charge at the scene, Brian Skotnes, said.

“We’ve apprehended one suspect and are intensively seeking more information as well as other people,” Skotnes said.

“Our hypothesis is that it is criminals who have an agenda against other criminals, but we cannot rule out anything.”

The area was now safe, he added.

The incident comes nearly a day after reports of drone sightings near Oslo’s and Copenhagen’s airports.

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