A landslide at a massive waste site in Sri Lanka's capital killed at least 19 people, authorities said on Saturday, as emergency workers dug into the mountain of rubbish to save survivors buried in dozens of houses.

The 91-metre high rubbish dump at the northeastern edge of Colombo collapsed on Friday when a fire broke out at its top, engulfing the mountain of rubbish in flames and triggering a landslide that swamped the homes below.

A 13-year-old boy and two girls aged 14 and 15 were among the dead, said Pushpa Soysa, head nurse at the main Colombo hospital.

Three other people pulled from the wreckage of their homes were already deceased by the time they arrived at the hospital, she said.

Another four children - two boys and two girls aged between 11 and 15 - succumbed to their wounds overnight, Soysa said on Saturday.

"We remain on standby, some people who were pulled out of wrecked homes were brought in overnight," she told the AFP news agency.

The fire broke out as the country marked its traditional Sinhala and Tamil New Year, and most people were in their houses celebrating.

Sri Lanka's government deployed hundreds of troops and police to dig through mounds of rubbish for survivors.

More than 600 people have fled their homes. Estimates ranged from 40 to 100 homes swamped by the fallen rubbish.

Police spokesman Priyantha Jayakody said police were investigating whether the collapse was a natural disaster or a deliberate act of sabotage.

Roughly 800 tonnes of solid waste is added daily to the 23 million tonnes of rubbish rotting at the open dump at Kolonnawa on the northeastern edge of the capital.

Residents in the area have been demanding the removal of the dump, saying it was causing health issues. The government had planned to clear it soon under a new infrastructure plan.