A massive bombing attack has hit government buildings in central Oslo on Friday.

At least seven people were killed and more than ten people were seriously injured by the explosion.

The blast broke most windows on the 17-story building housing Prime Minister Jens Stoltenberg's office.  The nearby oil ministry building was also set ablaze by the bomb. 

All roads entering the city center have been closed, reported Norwegian media NRK, and security officials evacuated people from the location in anticipation of another possible explosion.

A U.S. official said it was too soon to tell whether or not it was a terrorist attack. Terrorism is always a concern in Europe because of the continuous threat from al-Qaeda and possibly agents of Libyan strongman Muammar Gaddafi.