"And then it makes it higher", said Nazli Ismail, a geophysicist at the University of Syiah Kuala in Banda Aceh on Indonesia's Sumatra island, where a magnitude-9.1 natural disaster spawned a tsunami in 2004, killing 230,000 people in a dozen countries.
Some people climbed six metre (18 feet) trees to escape the tsunami and survived, he said.
Dramatic video footage captured from the top floor of a parking ramp as the tsunami rolled in showed waves bringing down several buildings and inundating a large mosque.
Photographs from the city on the coast of Sulawesi island showed bodies being lined up along the street, some in bags and others with their faces covered with clothes.
The tsunami smashed into buildings and a large mosque that had already been damaged in the quake. They were being treated outdoors due to continuing strong aftershocks, and many residents in the area were also sleeping outside, too afraid to return indoors. People are encouraged to gather in safe areas.
He said: "There was no time to save ourselves".
A key access road had been badly damaged and was partially blocked by landslides, the disaster agency said.
The head of Indonesia's search and rescue agency Muhammad Syaugi told local online media Detik.com that he could hear people calling out from the collapsed eight-story Roa-Roa Hotel in Palu.
Nugroho said that the casualties and the damage could be greater along the coastline 190 miles north of Palu in a regency called Donggala, which is closer to the epicenter of the quake.
Sutopo said they had not "got any information out of Donggala" yet.
Terrifying footage has emerged of the tsunami hitting Palu, which has a population of more than 380,000.
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"We're now getting limited communications about the destruction in Palu city, but we have heard nothing from Donggala and this is extremely worrying", he said. "There are more than 300,000 people living there", the Red Cross said in a statement, adding that its staff and volunteers were heading to the affected areas.
The current death toll is 420, but rescue teams are now scrabbling to reach the area and it is expected to rise into the thousands.
Rescue teams in Indonesia struggled on Sunday to reach communities feared devastated by a major natural disaster and tsunami on Sulawesi island.
Indonesia's meteorological and geophysics agency BMKG issued a tsunami warning after the quake, but lifted it 34 minutes later. Officials said they were rushing to reopen the airport to allow aid to come in.
Communications in the area have been cut due to power outages, hampering rescue and clean-up efforts.
The city's airport has been reopened only for relief efforts and will remain closed until October 4 for commercial flights, Nugroho said.
Nugroho said the runway of Palu's airport is not damaged and essential aircraft can land there.
President Joko Widodo was scheduled to visit evacuation centres on Sunday.
Indonesia sits on the seismically Pacific Ring of Fire and is regularly hit by earthquakes.
This latest quake comes after a series of earthquakes in July and August killed at least 623 people and destroyed hundreds of thousands of buildings on the Indonesian island of Lombok, which is situated hundreds of kilometres south-west of Sulawesi.
Palu, which has more than 380,000 people, was strewn with debris from the natural disaster and tsunami.