COLOMBO - Hopes of finding anyone alive under a collapsed mountain of garbage in Sri Lanka's capital faded on Sunday as the death toll reached 23 with another six reported missing, police said.

Hundreds of soldiers, backed by heavy earth moving equipment were digging through the rubbish and the wreckage of some 145 homes that were destroyed when a side of the 90-meter-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 six people 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 23 people killed. Hospital spokeswoman Pushpa Soysa said a man and a woman pulled out of the dump on Friday were in intensive care while another 11 were also still in hospital.

Authorities on Sunday held the funerals of 15 victims. Organizer of the "Movement Against the Meetotamulla Garbage Dump", Nuwan Bopage, said hundreds of people had attended the funeral to pay their final respects and offer their condolences to grieving family members who had lost their loved ones in the tragedy.

Bopage added that the funeral of the remaining victims would be held on Monday.

"The residents of Meetotamulla are extremely angry at the authorities for their failure to address this issue before. We have been urging the state to look into this matter for the past six years but no one did anything and today many lives have been lost," Bopage said.

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

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

Prime Minister Ranil Wickremesinghe, who is visiting Japan, said arrangements had been made to remove the garbage 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 tons of solid waste is added per day to the open dump.

Police said a total of 145 homes, mostly shacks, were destroyed when the garbage 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 tons of garbage rotting at Kolonnawa was a serious health hazard.

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