A large mud- and land slide hit the town of Ask, the administrative center of Gjerdrum municipality in southeastern Norway, early on Wednesday, police and local media said.
Police reported at least 10 injured, one of them critically.
A total of 26 people are unaccounted for — the number of people who have an address at the landslide site, police said.
Some 500 people were evacuated, according to local reports.
A 'serious' situation
"Police are designating this as a disaster," Pettersen told broadcaster NRK.
Emergency calls had come in from people in the Gjerdrum municipality, home to 5,000 people, saying their whole house was moving, Pettersen said. "So there are dramatic reports and the situation is serious."
"Conditions are challenging. It is dark and the weather is bad," Pettersen said, adding with more daylight expected soon, assessing the situation would be easier.
Nowegian rescue group Norsk Folkehjelp Follo said they had sent a total of 40 crew and 9 emergency ambulances to assist with search and rescue efforts.
Prime minister reacts
Norway's Prime Minister Erna Solberg offered her sympathies, saying: "It hurts to see how the forces of nature have ravaged Gjerdrum. My thoughts go to everyone affected by the landslide."
"This should have been a New Year's weekend where we should have had peace and quiet and maybe should have worried most about COVID-19 and not whether we have missing persons from a landslide," Solberg told Nowegian broadcaster TV2.
Quick clay region
The area where Ask is located is known for having a lot of so-called quick clay — a form of clay that can change from solid to liquid form. There have been previous landslides reported in the area.