"There are many dead and injured. The numbers I have for the time being are at least 12 killed, but this number is not definitive," Lieutenant Colonel John Espen Lien said, adding that no Norwegian soldiers in NATO's International Security Assistance Force (ISAF) were nearby when the blast occurred.
Most of the foreign forces in the Faryab province are Norwegians.
Whether the victims were civilians or military personnel was not immediately clear.
Faryab provincial authorities and a hospital doctor in the provincial capital Maymana meanwhile said at least five people had been killed and 26 others injured in the attack near a park in the city.
Northern Afghanistan has generally been spared this kind of attack, which have been common in the restive southeast of the country and are most often attributed to the Taliban.
Norway contributes a little over 400 soldiers to the 130,000-strong NATO-led ISAF force fighting a decade-long Taliban insurgency against President Hamid Karzai's Western-backed government.
The Scandinavian country, which has lost 10 soliders in Afghanistan since international forces invaded in late 2001, plan to withdraw most of its troops from the country next year.