"The cut to telecommunications and darkness are hampering efforts to obtain information," he said. "All national potential will be deployed, and tomorrow morning we will deploy Hercules and helicopters to provide assistance in tsunami-affected areas."
Sky TV's Siobhan Robins reports, warnings were issued before the tsunami hit.
"The big question now is how many people were still in their houses when those waves crashed onto the shore... And how many had actually made it to safer ground."
Officials have asked people to remain on the alert amid a series of moderate aftershocks.
"We advise people to remain in safe areas, stay away from damaged buildings," Sutopo Purwo Nugroho, spokesman for the National Disaster Mitigation Agency, said.
The agency was having difficulties reaching some authorities in the fishing town of Donggala and Palu city, the capital of central Sulawesi province, closest to the epicentre of the quake 80km away at a shallow 10km underground.
Palu airport was closed.
The US Geological Survey put the magnitude of the second quake at a strong 7.5, after first saying it was 7.7.
The earlier quake destroyed some houses, killing one person and injuring at least 10 in Donggala, authorities said.
More than 600,000 people live in Donggala and Palu.
"The (second) quake was felt very strongly, we expect more damage and more victims," Nugroho said, adding that evacuation process is still ongoing.
A series of earthquakes in July and August killed nearly 500 people on the holiday island of Lombok, hundreds of kilometres southwest of Sulawesi.
- with AP
