"The jolt was felt in Buton island and waters around it rose three to four metres but we don't know whether it has swept into the island," he told Reuters.
The quake struck at around 8.00 am (0000 GMT).
In one part of Buton island, women and children wept as they fled to an old palace on a nearby hill, leading news channel Radio El Shinta said. It was unclear how many people live on the island.
A rescuer later told Radio El Shinta that no one was injured due to the quake which was followed by dozens of mild tremors.
"Up until now, there has been no injuries nor severe damage to houses in Baubau (Buton's main city)," regional chief rescuer Rocky Asikin said.
"However, we are telling people to keep their calm as there is no tsunami. They fled at once without orders when it jolted as they thought they would face something like Aceh," he said.
The tremor follow a massive earthquake of magnitude 9.0 off Indonesia's Aceh province on Sumatra island on December 26 that triggered huge tsunami waves that left widespread devastation.
As many as 300,000 people around the Indian Ocean area are feared to have died in the disaster. More than 240,000 are dead or listed as missing in Aceh, where hundreds of bodies are still be recovered every day almost eight weeks later.
That quake has spawned hundreds of aftershocks and bred fear of tsunamis across Indonesia, one of the most earthquake-prone countries in the world.
The US Geological Survey Web site said Saturday's earthquake was measured initially at a magnitude of 6.5.