A loud blast was heard near the embassy compound in western Oslo at around 1 a.m., and smoke was seen rising from the compound, eyewitnesses told local media.
"We've determined that an explosion hit the American embassy," police spokesperson Mikael Dellemyr told public broadcaster NRK, adding that the blast occurred at the entry to the consular section.
It was not immediately clear what caused the blast or who was involved, the Oslo police department said in a separate statement.
"The police are in a dialog with the embassy, and there are no reports of any injured persons," the statement said.
Police are currently searching for the person responsible for the explosion.
