At least six people were killed and 10 injured in Sri Lanka's capital on Friday after a massive rubbish dump buried an estimated 40 homes. 

A 12-year-old boy and two teenage girls died at the Colombo National hospital where 10 others were being treated after they were rescued, hospital spokeswoman Pushpa Soysa said. 

Three other people who were pulled from the wreckage of homes were already dead by the time they arrived at the hospital on Friday night, raising the confirmed death toll to six.

Police said hundreds of troops had joined the search for survivors after the disaster at Kolonnawa on the northeastern edge of the capital Colombo. 

Officials said that President Maithripala Sirisena ordered troops and police to join firefighters in the rescue after the 300-foot high dump caught fire and collapsed.

The police said in a statement "A search for survivors is under way,"

Officials said dozens of homes collapsed after heavy rain overnight caused the garbage mountain to shift. It became further destabilised after a fire broke out, triggering landslides that buried dwellings.

Military spokesman Roshan Seneviratne said 100 soldiers were already digging through mounds of trash. Heavy earth moving equipment was also being deployed. 

Local residents said many people had left the area after the night's heavy rain. A disaster management official said "We think about 40 homes have been destroyed."

It's believed that around 800 tonnes of solid waste is added daily to the open dump, angering those who live nearby.