PALU, Indonesia - The death toll from an earthquake and tsunami on the Indonesian island of Sulawesi rose to 832 on Sunday, the national disaster mitigation agency said, adding it assessed the affected area to be bigger than initially thought.

Many people were reported trapped in the rubble of buildings brought down in the 7.5 magnitude earthquake which struck on Friday and triggered tsunami waves as high as six metres (20 feet), agency spokesman Sutopo Purwo Nugroho told a news conference. 

Dozens of people were reported to be trapped in the rubble of a hotel and a mall in the city of Palu, which was hit by waves as high as six metres (20 feet) following the 7.5 magnitude earthquake on Friday.

A young woman was pulled alive from the rubble of the Roa Roa Hotel, the news website Detik.com reported. Hotel owner Ko Jefry told Metro TV on Saturday that up to 60 people were believed trapped. Hundreds of people gathered at the mall searching for loved ones.

"We've got information from people that their relatives are still inside, so we’re focusing on that, especially to find survivors," a rescuer identified as Yusuf, working at the ruins of the mall, told Metro TV.

With confirmed deaths only from Palu, authorities are bracing for much worse as reports filter in from outlying areas, in particular, Donggala, a region of 300,000 people north of Palu and closer to the epicentre of the quake.

Vice President Jusuf Kalla said the toll could rise into the thousands.

A disaster official said the tsunami travelled across the open sea at speeds of 800 kph (500 mph) before striking the shore and casualties could have been caused along a 300 km (200 miles) stretch of coast, north and south of Palu.

The fishing town of Donggala has been extensively damaged, with houses swept into the sea and bodies trapped in debris, according to a Metro TV reporter on the scene.

"We have heard nothing from Donggala and this is extremely worrying. There are more than 300,000 people living there," the Red Cross said in a statement.

"This is already a tragedy, but it could get much worse."