An explosion has killed at least two people outside offices occupied by the prime minister in Oslo, Norway.

The offices of Norwegian prime minister Jens Stoltenberg were extensively damaged following what officials described as a “bomb”.

Stoltenberg was reportedly unharmed following the incident.

No terrorist group has so far claimed responsibility for the attack.

However, the British Foreign & Commonwealth Office (FCO) warned travellers to the Scandinavian country to exercise caution.

“There is an underlying threat from terrorism,” warned an FCO statement.

“Attacks, although unlikely, could be indiscriminate, including in places frequented by expatriates and foreign travellers.”

All roads into the city centre have been closed following the attack.

Local people are also being evacuated from the area by security personnel fearing a second blast.

“Police can confirm there were deaths and injuries following the explosion in the government quarter this afternoon,” police said in a statement.