The police chief declined to comment about the exact number of deaths and injuries, but reports suggest multiple people have been killed or injured.
Mr Aas added there were “several crime scenes” across the city.
The Norwegian Minister of Justice and Public Security Monica Maeland has received updates from the incident and is closely monitoring the situation according to the ministry.
The country’s police directorate said it had immediately authorised all officers, who are normally unarmed, to arm themselves with weapons.
In a statement, the directorate said: “This is an extra precaution. The police have no indication so far that there is a change in the national threat level.”