The landslide hit a residential area in the municipality of Gjerdrum, about 30 kilometers (19 miles) north of the capital Oslo. Police said more than 900 people were evacuated from the area.
Rescuers continued to search the area for children and adults who feared they had been caught in the mud and rubble, police said.
“We are sure there are people in the affected area, but we do not know if all 11 are there or if the number is less,” police spokesman Roger Pettersen told a news conference.
“We are still looking for survivors,” he said.
Photos from the site showed a large crater with destroyed buildings at the bottom. Other buildings hanging from the edges of the crater showed TV images. Two more houses collapsed in the crater on Wednesday afternoon, NRK reported.
King Harald of Norway said the landslide impressed him deeply.
“My thoughts are with all those who have been affected, injured or lost their homes and those who now live in fear and in the uncertainty of the full extent of the catastrophe,” said the 83-year-old monarch in a statement released by royalty Palace.
Helicopters continued to hover over the area as night fell, sometimes lowering emergency crews to the wreckage of collapsed houses.
“There may be people arrested … but at the same time we cannot be sure because it is a New Year’s holiday, which means that people may be elsewhere,” Prime Minister Erna Solberg told reporters after visiting the site.
Local residents spoke of their experience.
“There were two massive tremors that lasted a long time and I assumed it was snow being removed or something,” Oeystein Gjerdrum, 68, told NRK.
“Then the light suddenly fell, and a neighbor came to the door and said that we needed to evacuate, so I woke up my three grandchildren and told them to get dressed quickly.”
Land masses are still moving on what has been one of the biggest clay slides in Norway’s recent history, Toril Hofshagen, regional head of the Norwegian Water and Energy Directorate, told a news conference.