Workers were digging a mass grave Sunday in an Indonesian city hard-hit by a powerful earthquake and tsunami, and officials said it would hold at least 300 bodies.

Indonesia's National Disaster Mitigation Agency said the grave in Palu on Sulawesi island would be expanded if needed and the burial would take place as soon as possible for health and religious reasons. The majority of Palu's inhabitants are Muslim and would prefer funeral rites within 24 hours.

Rescuers have been scrambling 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 the 7.5 magnitude earthquake spawned the deadly tsunami on Friday evening.

Indonesian rescuers try to free a 15-year-old earthquake survivor, Nurul Istikhomah, from the flooded ruins of a collapsed house in Palu on Sunday. (Erimacs Wilander/EPA-EFE)

The quake created a tsunami that formed a wall of water as high as six metres at dusk Friday. More than 830 people have been confirmed dead, but officials fear that toll could climb much higher.

Indonesia's Vice President Jusuf Kalla said the death toll could rise into the thousands.

A man searches for victims among the rubble of a 10-storey hotel in Palu in Indonesia's Central Sulawesi on Sunday. (Bay Ismoyo/AFP/Getty Images)

A village chief on Sunday said 100 to 200 people could be buried under the debris of a residential complex in Palu.

Two days after the twin disaster, people could still be heard calling out from what remains of the eight-storey Roa-Roa Hotel, said Muhammad Syaugi, 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.

The rescue agency released images of a 25-year-old woman it identified as Fitri, who it said was pulled out of the rubble of the hotel at 7 p.m. local time Sunday.

Elsewhere in the city, rescue workers were working to free a 15-year-old earthquake survivor, Nurul Istikhomah, from the flooded ruins of a collapsed house in Palu. She has been trapped under heavy concrete under water for two days.

Most of the dead were from Palu. The city of Mamuju was also ravaged, but little information was available due to damaged roads and disrupted telecommunications.

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.

Damage to the Mutiara Sis Al-Jufri airport is seen in Palu on Sunday. (Adek/AFP/Getty Images)

Indonesia's disaster agency spokesperson Sutopo Purwo Nugroho said "tens to hundreds" of people were taking part in a beach festival in Palu when the tsunami struck. Their fate was unknown.

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

Indonesian President Joko "Jokowi" Widodo arrived in Palu on Sunday and said the city's airport "is not 100 per cent ready for landing."

He said rescue teams were having difficulty recovering victims because of a shortage of heavy equipment. He said authorities were deploying more heavy machinery that he hoped would arrive Sunday night so emergency workers can help recover more victims on Monday.

'No time to save ourselves'

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.

A man walks inside a shopping mall damaged by earthquakes and a tsunami in Palu. (Tatan Syuflana/Associated Press)

"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.

An aerial view shows bridge damaged by an earthquake and tsunami in Palu. (Antara Foto/Muhammad Adimaja via Reuters)

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."

Humanitarian aid

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 six metres high," said Nugroho, the disaster agency spokesperson.

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.

This mosque in Palu was damaged after the earthquake and tsunami. (Tatan Syuflana/Associated Press)

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.

Damaged cars are seen in Palu. (Bay Ismoyo/AFP/Getty Images)

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

Sulawesi has a history of religious tensions between Muslims and Christians, with violent riots erupting in the town of Poso, not far from Palu, two decades ago. Indonesia is home to the world's largest Muslim population.

With files from Reuters