No fewer than 254 people are now confirmed dead after severe flooding and landslides hit the city of Mocoa in south-western Colombia.
Of the the number, authorities say 60 were children.
More than 200 people are still missing.
Water and sludge flowed into 17 of Mocoa’s 40 neighbourhoods early Saturday, sweeping away houses or burying them under debris, after heavy rainfall caused the Mocoa, Mulato and Sancoyaco rivers to burst their banks.
Santos cancelled a trip to Cuba in order to go to the disaster area and ordered soldiers to deploy to the region.
After his first visit on Saturday, Santos returned to Mocoa on Sunday with several ministers.
Mocoa, which has a population of 40,000, is near the Ecuadorean border, around 630 km south-west of Colombia’s capital Bogota.