Disaster agency spokesman Sutopo Purwo Nugroho said officials are trying to confirm the number still missing in several villages obliterated when the quake caused loose soil to liquefy, sucking houses into deep mud and burying occupants.
He said: “Based on reports from village chiefs in Balaroa and Petobo, some 5,000 people have not been found. Our workers on the ground are trying to confirm this.”
Mr Nugroho said efforts to retrieve decomposed bodies in deep, soft mud were getting more difficult and that some people may have fled or been rescued and evacuated.
More than 8,000 either injured or vulnerable residents have been flown or shipped out of Palu, while others could have left by land, he said.
Officially, Mr Nugroho said only 265 people are confirmed missing and 152 others still buried under mud and rubble, nine days after the magnitude 7.5 earthquake and powerful tsunami hit Palu and its surrounding areas.
The government targets to end search operations by Thursday, nearly two weeks after the disaster, at which time those unaccounted for will be declared missing and considered dead, Mr Nugroho said.
The scale of the death toll has left authorities considering turning devastated areas into mass graves to accommodate the rising death toll.
Aid has begun pouring into central Indonesia's Sulawesi island and humanitarian workers are fanning out across its countryside, more than a week after parts of the island were devastated by a powerful earthquake and tsunami.
The military dropped supplies from helicopters in places and a large Red Cross ship docked at a port in the region. In the dusty one-road village of Pewunu, excited children shouted "Red Cross! Red Cross!" as one of the aid group's medical teams arrived.