The tremor struck late on Monday night while most residents in the affected regions slept. Initial reports suggested that as many as 2000 people could have died in the Indonesian island of Nias alone.
"Maybe there are one to two thousand people who have died according to the early reports from there," Indonesian Vice-President Jusuf Kalla said.
The island's main town has suffered serious damages.
"We predict that up to 75% of Gunungsitoli town has been badly damaged ... at least for now 70% of the buildings have collapsed," a local police official said.
"Gunungsitoli is now like a dead town," said another official, confirming that dozens of people had been killed.
"About the victims, we cannot count them now. We only know there are many buildings flattened and a big possibility there will be hundreds of people killed," Zulkifly Sirait, a police sergeant said.
The epicentre of the quake, off the coast, was very close to that of the 26 December quake which triggered a tsunami that left nearly 300,000 people dead or missing across Asia.
The tremor spread panic across western Indonesia, Sri Lanka and coastal parts of Malaysia and Thailand - the areas devastated by the 26 December tsunami.
Sirens wailed and tens of thousands of people were evacuated by tsunami warnings while others drove or ran from the coast to higher ground.
There were no signs of a tsunami up to three and a half hours after the 1609 GMT quake.
Nias, off the western coast of Sumatra island and about 1400km northwest of Jakarta, is a remote and rugged island famed as a surfing paradise.
The Pacific tsunami warning centre said the quake had the potential to cause a "widely destructive tsunami" and the authorities should take immediate action, including evacuating coastlines within 1000km of the epicentre.
Unlike in the immediate aftermath of the December quake, reactions were quick across nations this time round.
Tens of thousands of people fled their homes in Sumatra. Thailand urged people living along parts of its west coast to evacuate while Malaysia issued a warning to coastal residents.
The authorities in India's Andaman and Nicobar islands issued a preliminary tsunami warning, before withdrawing it several hours later. Sri Lanka also sounded a tsunami alert for its coastal residents.
A spokesman for the US Geological survey said the quake struck 1400km northwest of Indonesian capital Jakarta.
The tremor was felt as far away as Singapore and the Malaysian coastal city of Penang.