Hopes of finding anyone alive under a collapsed mountain of rubbish in Sri Lanka's capital faded on Sunday as the death toll reached at least 23, police said.

Hundreds of soldiers were digging through the rubbish and the wreckage of some 145 homes that were destroyed in northeast Colombo when a side of the 90-metre high dump crashed on Friday.

"The rescue is fast becoming a recovery operation," a senior police official at the site said. "It is difficult to imagine anyone could survive under these toxic conditions."

He said a woman and three men were reported missing after Friday's disaster at Kolonnawa on the northeastern edge of the capital.

The Colombo National hospital said four children aged between 11 and 15 were among the 21 people killed.

Hospital spokeswoman Pushpa Soysa said a man and a woman pulled out of the dump on Friday were still in intensive care.

Reports of looting

Police have stepped up security in the area following reports of looting and said they arrested 18 people suspected of robbing belongings of the victims.

President Maithripala Sirisena ordered hundreds of troops to search for survivors and bolster the fire department's rescue efforts.

Prime Minister Ranil Wickremesinghe, who is currently visiting Japan, said arrangements had been made to remove the rubbish dump, but it came crashing down before relocation work could begin.

Wickremesinghe said Japanese Prime Minister Shinzo Abe offered help with the recovery effort and a technical team would be sent to Sri Lanka to evaluate the situation.

About 800 tonnes of solid waste is added a day to the open dump.

Police said a total of 145 homes, mostly shacks, were destroyed when the rubbish mountain came crashing down following heavy rain the previous day and a fire hours earlier.

More than 600 people have been given temporary shelter at a government-run school in the area, as authorities looked for alternative accommodation for those living near the dump.

Many residents had evacuated their homes before the disaster because of the heavy rain.

Sri Lanka's parliament was warned recently that the 23 million tonnes of rubbish rotting at Kolonnawa was a serious health hazard.

Efforts are under way to generate electricity using solid waste as fuel.