At least 206 people were killed and 202 injured by landslides in Colombia's south-west Putumayo province.
Several homes were flooded with mud when rivers burst their banks following hours of heavy rain.
At least 220 people have gone missing, the Red Cross said. Relief operations are ongoing.
President Juan Manuel Santos has declared a state of emergency in the region.
How did the landslides occur?
An official said heavy rainfall led to a rise in water levels of the Mocoa river and three tributaries, triggering landslides. Landslides and heavy rains are common in the mountainous region. However, March was Colombia's wettest month since 2011, which could explain the landslides' severity.
Local infrastructure decimated by landslides, complicates rescue efforts
Putumayo's governor said entire neighbourhoods were buried by the landslides.
"There are mobility issues on almost 80% of the roads, and from where the road ends, it is three hours to where the landslide took place," one police officer said.
Damaged infrastructure along with inclement weather have hindered rescue efforts.
An official said the provincial capital Mocoa is "totally isolated", without electricity and water.
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Colombia: FARC rebels offer to help rebuild landslide affected town
However, the government has yet to approve FARC's offer of help. Currently, over 1000 policemen and military personnel are involved in rescue efforts in affected regions.
FARC rebels who are in transition camps under the peace deal made the offer.
Juan Manuel Santos
President Juan Manuel Santos