PALU, Indonesia -- Bright-colored body bags were placed side-by-side in a freshly dug mass grave Monday, as a hard-hit Indonesian city began burying its dead from the devastating earthquake and tsunami that killed more than 840 people and left thousands homeless. As CBS News correspondent Ben Tracy reports, the death toll was expected to rise significantly given how difficult it has been for rescuers to reach some of the hardest-hit areas, where hundreds more are feared dead.

The seismic shock from the magnitude 7.5 quake triggered a tsunami nearly 20-feet-high, which came roaring over the island of Sulawesi without warning after the earthquake cut power to the tsunami alert system, reports Tracy.

Until Sunday night, cries for help could be heard from the rubble of the Roa Roa Hotel in the devastated city of Palu, where most of the deaths have been confirmed thus far. Officials believe as many as 50 people could still be buried under the mountain of debris.

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But while some crews were still racing to save survivors on Monday, others had turned to the grim task of collecting bodies.

Local Army Commander Tiopan Aritonang said 545 bodies would be brought to the newly-dug mass grave from one hospital alone, but that only some would be buried on Monday. The grave being dug in Palu will be 33 feet by 330 feet and could be enlarged if needed, said Willem Rampangilei, chief of Indonesia's National Disaster Mitigation Agency.

"This must be done as soon as possible for health and religious reasons," he said. Indonesia is majority Muslim, and religious custom calls for burials soon after death, typically within one day.

Local military spokesman Mohammad Thorir said the area adjacent to a public cemetery on a hill can hold as many as 1,000 bodies. All the victims, coming from local hospitals, have been photographed to help families locate where their relatives were buried.

Some 1,200 inmates escaped from three detention facilities on Sulawesi after Friday's quake and tsunami, French news agency AFP reported Monday, citing a justice ministry official. AFP quoted Sri Puguh Utami as saying the inmates fled from two facilities in the hard-hit city of Palu, both of which were overcapacity, and one in Donggala.

"I'm sure they escaped because they feared they would be affected by the earthquake. This is for sure a matter of life and death for the prisoners," she said. The facility in Donggala was set on fire and all 343 inmates were still on the loose, Utami said. Most of the escapees were behind bars for corruption and narcotics offenses, she added.

Rescue teams evacuate a victim from the Roa-Roa Hotel in Indonesia on Sept. 30, 2018. Antara Foto Agency/REUTERS

Tracy reports that with emergency crews focused entirely on the search and rescue effort, there are no police available to try and track down the escapees.

A 25-year-old woman was found alive Sunday evening in the ruins of the Roa-Roa Hotel, according to the National Search and Rescue Agency, which released photos of the her lying on a stretcher covered in a blanket. A number of other survivors were still being found and a few were being pulled from buildings in different locations.

The cries from beneath the hotel, which appeared to have toppled over with its walls splintered like pickup sticks, went silent by Sunday afternoon.

"We are trying our best. Time is so important here to save people," said Muhammad Syaugi, head of the national search and rescue team. "Heavy equipment is on the way."

Metro TV showed about a dozen rescuers in orange jumpsuits climbing over debris with a stretcher carrying the body of a victim from the modest business hotel.

Other rescuers worked to try to free a 15-year-old girl trapped under concrete in her house in Palu after it collapsed on her family during the earthquake. Unable to move her legs under the rubble, Nurul Istikharah was trapped beside her dead mother and niece. Rescuers also tried to control water from a leaking pipe, fearing she would drown.

Istikharah was unconscious during part of the effort to free her, but rescuers kept talking to her to try to keep her awake. Others offered her food and water.

Indonesian President Joko "Jokowi" Widodo authorized the acceptance of international help, Thomas Lembong, chair of Indonesia's Investment Coordinating Board, tweeted Monday morning. It wasn't immediately clear what type of help was approved, but the stricken areas needed medical supplies, fuel, fresh water and experts.

"We will send food today, as much as possible with several aircraft," he told journalists in the capital, Jakarta, adding a supply of fuel was also set to arrive.

In Donggala, the site closest to the earthquake's epicenter, aerial footage on Metro TV showed the sugary blond sands of beaches swept out to sea, along with some buildings. Some structures in the town were severely damaged, with plywood walls shredded and chunks of concrete scattered on the pavement. Much of the damage, however, appeared limited to the waterfront.

Palu, which has more than 380,000 people, was strewn with debris from the earthquake and tsunami. A heavily damaged mosque was half-submerged and a shopping mall was reduced to a crumpled hulk. A large bridge with yellow arches had collapsed.

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

In Palu, residents said dozens of people could still be buried in their homes.

"The ground rose up like a spine and suddenly fell. Many people were trapped and buried under collapsed houses. I could do nothing to help," resident Nur Indah said, crying. "In the evening, some of them turned on their cellphones just to give a sign that they were there. But the lights were off later and the next day."

With hundreds injured, earthquake-damaged hospitals were overwhelmed.

Nugroho said 61 foreigners were in Palu at the time of the disaster. Most were accounted for, but one South Korean was believed to be trapped in the Roa-Roa Hotel, while three others from France and one from Malaysia were missing.

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

The earthquake/tsunami was 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. More recently, a powerful quake on the island of Lombok killed 505 people in August.

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