Rescuers in Indonesia were scrambling Sunday to reach trapped victims screaming for help from collapsed buildings, while looters risked entering an unstable shopping mall to grab whatever they could find after a massive earthquake spawned a tsunami that left more than 800 dead.

People could still be heard calling out from the eight-story Roa-Roa Hotel which toppled as Friday's twin disasters swept through the hard-hit city of Palu on the island of Sulawesi, said Muhammad Syaugi, the head of Indonesia's search and rescue agency.

"I can still hear the voices of the survivors screaming for help while inspecting the compound," he told local online news portal Detik.com, adding there could be 50 people trapped inside.

Disaster agency spokesman Sutopo Purwo Nugroho told a news conference Sunday that the death toll had more than doubled to 832, with nearly all of those killed in Palu. He said there were still not comprehensive reports about casualties in surrounding coastal areas.

"The death is believed to be still increasing since many bodies were still under the wreckage, while many have not been reached," he said.

Aid and supplies were being sent in via military and commercial aircraft, including helicopters, to reach badly affected areas.

The nearby cities of Donggala, the site closest to the earthquake's epicenter, and Mamuju were also ravaged, but little information was available due to damaged roads and disrupted telecommunications.

Footage from MetroTV on Sunday showed images of destroyed houses in Donggala and areas that were once land now inundated with water. Aerial video also showed the battered coastline surrounding Palu.

Looters were stealing Sunday from a badly damaged mall in Palu that was not being guarded. They did not appear to be concerned about their safety, despite ongoing aftershocks and the structure's questionable stability. Residents were also seen returning to their destroyed homes, picking through waterlogged belongings, trying to salvage anything they could find.

Nugroho said "tens to hundreds" of people were taking part in a beach festival in Palu when the tsunami struck at dusk on Friday. Their fate was unknown.

Hundreds of people were injured and hospitals, damaged by the magnitude 7.5 quake, were overwhelmed.

Indonesian President Joko "Jokowi" Widodo visited the area on Sunday.

Some of the injured, including Dwi Haris, who suffered a broken back and shoulder, rested outside Palu's Army Hospital, where patients were being treated outdoors due to continuing strong aftershocks.

Tears filled his eyes as he recounted feeling the violent earthquake shake the fifth-floor hotel room he shared with his wife and daughter.

"There was no time to save ourselves. I was squeezed into the ruins of the wall, I think," said Haris, adding that his family was in town for a wedding. "I heard my wife cry for help, but then silence. I don't know what happened to her and my child. I hope they are safe."

It's the latest natural disaster to hit Indonesia, which is frequently struck by earthquakes, volcanic eruptions and tsunamis because of its location on the "Ring of Fire", an arc of volcanoes and fault lines in the Pacific Basin.

In December 2004, a massive magnitude 9.1 earthquake off Sumatra island in western Indonesia triggered a tsunami that killed 230,000 people in a dozen countries. Last month, a powerful quake on the island of Lombok killed 505 people.

Palu, which has more than 380,000 people, was strewn with debris from the earthquake and tsunami.

A mosque heavily damaged by the quake was half submerged and a shopping mall was reduced to a crumpled hulk. A large bridge with yellow arches had collapsed. Bodies lay partially covered by tarpaulins and a man carried a dead child through the wreckage.

The city is built around a narrow bay that apparently magnified the force of the tsunami waters as they raced into the tight inlet.

Indonesian TV showed dramatic smartphone video of a powerful wave hitting Palu, with people screaming and running in fear. The water smashed into buildings and the mosque.

Nina, a 23-year-old woman who goes by one name, was working at a laundry service shop not far from the beach when the quake hit. She said the quake destroyed her workplace, but she managed to escape and quickly went home to get her mother and younger brother.

"We tried to find shelter, but then I heard people shouting, 'Water! Water!'" she recalled, crying. "The three of us ran, but got separated. Now I don't know where my mother and brother are. I don't know how to get information. I don't know what to do."

The earthquake left mangled buildings with collapsed awnings and rebar sticking out of concrete like antennae. Roads were buckled and cracked. The tsunami created even more destruction.

"We got a report over the phone saying that there was a guy who climbed a tree up to 6 meters high," said Nugroho, the disaster agency spokesman.

Communications with the area were difficult because power and telecommunications were cut, hampering search and rescue efforts. Most people slept outdoors, fearing strong aftershocks.

"We hope there will be international satellites crossing over Indonesia that can capture images and provide them to us so we can use the images to prepare humanitarian aid," Nugroho said.

Indonesia is a vast archipelago of more than 17,000 islands that's home to 260 million people. Roads and infrastructure are poor in many areas, making access difficult in the best of conditions.

The disaster agency has said that essential aircraft can land at Palu's airport, though AirNav, which oversees aircraft navigation, said the runway was cracked and the control tower damaged.

AirNav said one of its air traffic controllers, aged 21, died in the quake after staying in the tower to ensure a flight he'd just cleared for departure got airborne safely. It did.

More than half of the 560 inmates in a Palu prison fled after its walls collapsed during the quake, said its warden, Adhi Yan Ricoh.

"It was very hard for the security guards to stop the inmates from running away as they were so panicked and had to save themselves too," he told state news agency Antara.

Ricoh said there was no immediate plan to search for the inmates because the prison staff and police were consumed with the search and rescue effort.

"Don't even think to find the inmates. We don't even have time yet to report this incident to our superiors," he said.

United Nations Spokesman Stephane Dujarric said UN officials were in contact with Indonesian authorities and "stand ready to provide support as required."