More than 250 have been killed in south west Colombia after a mud landslide buried homes while residents were asleep.

Four hundred people have been injured and 200 are still missing after the disaster in the city of Mocoa in the early hours of Saturday morning.

The army said in a statement at least 254 people have died but the number could still rise.

The race is on to reach survivors and more than 1,100 soldiers and police officers were called in to help dig people out in 17 affected neighbourhoods.

The incident was triggered by intense rains around midnight and muddy water and debris quickly surged through the city's streets.

Homes were toppled, trees ripped from their roots and cars and trucks carried downstream.

President Juan Manuel Santos arrived in the city on Saturday and declared a state of emergency. He also warned the death toll could rise.

"We don't know how many there are going to be," he said of the fatalities when he arrived at the disaster zone. "We're still looking," he added.

A surgeon at the local hospital said doctors are running out of blood.

Witnesses described how buildings vibrated and although an alarm reportedly went off to alert residents it could not be heard throughout the city.

Images posted on Twitter by the Colombian army showed vast areas filled with wood planks and debris.

One man said he had not heard anything from any family members, and had lost everything; even the clothes on his back were borrowed.

Another woman lost a family member who was stripped from her husband's grasp by the strong currents.

The disaster happened in the early hours of Saturday morning. Credit: Twitter / Army of Colombia
Heavy rain caused a river to burst its banks, triggering an avalanche of mud and water. Credit: Twitter / Colombian Military Forces

The Red Cross have set up a special unit in Mocoa to help relatives search for their loved ones.

"In this moment, it's chaos," said Oscar Forero, a spokesman with the Colombian Red Cross. "There are many people missing."

Mocao is a city in the province of Putumayo near Colombia's border with Ecuador and has a population of 40,000.

More than 250 people have been killed and the president warned the death toll could rise. Credit: Twitter / Colombian Military Forces