Video footage from the scene showed a whole hillside had collapsed in Ask in the municipality of Gjerdrum, 25 kilometres (15 miles) northeast of the capital, leaving homes crushed and buried in dark mud.
Photos of the area showed at least eight homes destroyed.
According to police, ten people were injured with one transferred to Oslo with serious injuries. There have been previous landslides reported in the area.
He said 21 people who were registered to live in the area were unaccounted for.
Of those still missing, police said in a statement: "We don't know if these people are in the landslide area, if they are away on holiday or in another way unable to contact police". Weather at the time was reported to be challenging, with snow and full winter conditions.
About 300 people who were evacuated from Ask took shelter at a hotel in nearby Skjetten, its manager said. Police said rescue operations would continue through Wednesday night into Thursday.
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Solberg said that the situation is still "so unstable" that it was impossible to do any rescue effort other than from helicopters. "Now it is important that the emergency services get their job done", Norwegian Prime Minister Erna Solberg tweeted.
At least six people from Ask were hospitalized and hundreds more from the village were evacuated.
According to the Norwegian Water Resources and Energy Directorate (NVE), around 700 have been evacuated.
Southern Norway has seen large amounts of precipitation in recent days, which may have caused the clay soil prevalent in the area to shift, broadcaster NRK said.
The evacuation has now slowed because the landslide cut straight across a road through the village, leaving a deep ravine which cars could not pass. My thoughts go to all those affected by the landslide.
"The area has been surveyed earlier, and is known to contain quick clay".