Rescuers are continuing the desperate search for hundreds of victims after after a mud landslide buried homes while residents were asleep in south-west Colombia.

Over 200 people have been killed in the disaster, hundreds more have been injured and at least 200 are still missing after heavy rain flooded the town of Mocoa in the early hours of Saturday morning.

Many of those killed after a torrent of water, mud and debris swept through the town were children.

Firefighters search for survivors after the mud slide in Mocoa, Colombia. Credit: AP Photo/Fernando Vergara

Neighborhoods were left strewn with rocks, wooden planks, tree limbs and brown muck after heavy rain caused the three rivers that surround Mocoa to rise up and surge through the city with a population of 40,000.

Search-and-rescue teams combed through the debris and helped people who had been desperately clawing at huge mounds of mud by hand.

"People went to their houses and found nothing but the floor," said Gilma Diaz, a 42-year-old woman from another town who came to search for a cousin.

President Juan Manuel Santos, who visited Mocoa for a second day on Sunday, declared the area a disaster zone and said the death toll stood at 210.

Survivors search for any remaining belongings after homes were destroyed. Credit: AP Photo/Fernando Vergara

Santos said more than 40 of the dead identified so far were under 18, perhaps because youngsters were already in bed when the floodwaters struck.

The death toll could still rise because authorities say there were over 200 people injured, some in a critical condition and over 200 still missing.

The president said on Twitter that 170 of the dead had so far been identified.

Maria Cordoba, a 52-year-old resident who was trying to wash her belongings in a river, said two of her nephews, ages 6 and 11, were killed when their house was destroyed.

"The mother as well was totally beaten up" but managed to save her 18-month-old baby, she said.

A man looks inside a passenger bus damaged in the flooding. Credit: AP Photo/Fernando Vergara

Not far away, Abelardo Solarte, a 48-year-old resident of Mocoa, held a child's shoe as he helped clear debris.

"You have no idea how many kids there are around here," Solarte said.

Jair Echarri, who came from a nearby town to help, also struggled to comprehend the loss of so many children.

"I feel an enormous sadness because it's filled with kids' things, toys, clothes, school books," he said.

"I am a father and this breaks my heart."

Colombian National Army soldiers rescue a child from the debris. Credit: Colombian National Army via AP

President Santos said the avalanche of water and debris knocked out power in half of the province of Putumayo, where Mocoa is located, and destroyed the area's fresh water network, creating dangerous and unsanitary conditions.

Mocoa is vulnerable to flooding as it is surrounded by the three rivers in a natural basin created by the surrounding mountains.

The danger has grown worse in recent years because of deforestation.

But the triggering event was over 5 inches of rainfall (130 millimeters) that began late on Friday.

A 1989 hydrology report for the Agricultural Ministry warned that just such a disaster could happen unless steps were taken to reinforce the riverbanks, channel water away from the town and restore some of the forest.

It was not immediately clear why those steps had not been taken.