Rescuers scrambled to find buried survivors on Saturday after a powerful earthquake on Indonesia's Sulawesi island killed dozens, injured hundreds and left more feared trapped in the rubble of collapsed buildings.

At least 45 people died after the 6.2-magnitude quake struck in the early hours of Friday, triggering panic among residents of the island, which was hit by a 2018 quake-tsunami disaster that killed thousands.

Dozens of bodies have been hauled from beneath crumpled buildings in Mamuju, a city of about 110,000 in West Sulawesi province, while others were killed south of the area after the quake struck.

“The latest data we have is 45 [dead]", said Arianto from the rescue agency in Mamuju, who like many Indonesians goes by one name.

The toll was up from 42 on Friday evening.

Authorities have not given a figure for the number of residents who could be trapped under levelled buildings, including a hospital that collapsed with more than a dozen patients and staff inside.

At least one hotel had partially collapsed, while the regional governor's office also suffered extensive damage.

About 15,000 residents have fled to temporary shelters and nearly 190 people were being treated for serious injuries, local authorities said.

'Heartfelt solidarity'

The Pope said he was “saddened” to learn of the quake.

“His Holiness Pope Francis expresses his heartfelt solidarity with all those affected by this natural disaster,” the Vatican said in a statement.

“He prays for the repose of the deceased, the healing of the injured and the consolation of all who grieve.”

Images from the scene on Friday showed residents trying to flee Mamuju in cars and motorbikes as they drove past corrugated metal roofs and other building debris scattered on the roadside.

But landslides triggered by heavy rains and the quake blocked the main access road out of the seaside city.

The meteorological agency warned residents that the area could be hit by strong aftershocks and to avoid the beachfront in case of a tsunami.

The city's airport had also been damaged, authorities said.

The Indonesian Red Cross said it was rushing medical and relief supplies to the scene, with its teams working to help find trapped residents.

Save the Children warned that the young were among the most at risk.

“While the extent of the earthquake damage is still unclear, we know children are often the most vulnerable following disaster,” it said.

“It will be essential that children are prioritised in any response, as they may have witnessed the death of loved ones or become separated from their parents.”

The quake's epicentre was 36 kilometres south of Mamuju and it had a relatively shallow depth of 18 kilometres.

Indonesia experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire", where tectonic plates collide.

In 2018, a 7.5-magnitude quake and a subsequent tsunami in Palu on Sulawesi left more than 4,300 people dead or missing.

On December 26, 2004, a 9.1-magnitude earthquake struck off the coast of Sumatra and triggered a tsunami that killed 220,000 throughout the region, including around 170,000 in Indonesia.