Indonesia issued a tsunami warning after a powerful earthquake struck off the Mentawai island chain on Wednesday.
The US Geological Survey (USGS) said a magnitude-7.9 earthquake struck some 660 kilometres south-west off the Indonesian island of Sumatra, at a depth of 10 kilometres.
It hit at 7:49 pm local time (1249 GMT), the agency said.
There were no reports of damage or casualties on Indonesia's Mentawai island chain, said Zulfiatno, the head of West Sumatra provincial disaster management agency, told Elshinta radio.
"We have contacted officials on Mentawai and they told us there was no damage or casualties," Zulfiatno said,
"But they remain on alert for a possible tsunami."
Indonesia's Meteorology, Geophysics and Climatology Agency issued the tsunami warning for the Mentawai island chain, which is located 150 kilometres off the western coast of Sumatra.
Parts of Sumatra including Aceh, Bengkulu, North Sumatra and Lampung provinces were at risk of being hit by a tsunami, the agency said.
"The likelihood of a stsunami is very small but people who live in coastal areas are not advised to return yet," said Titi Handayani of the Indian Tsunami Early Warning Centre.
Australia issued a tsunami warning for Christmas Island and Cocos Island.
In October 2010, a 7.7-magnitude quake triggered a tsunami off the Mentawai islands, which then had a population of 68,000 people. More than 500 people were killed, while tens of thousands were displaced.
The Mentawai chain comprises 70 islands and islets.
The world's most quake-prone regions are located around the Pacific Ocean from Japan down to the Philippines and Indonesia and in the Americas, from Alaska to Chile - named the Pacific Ring of Fire, where about 90 per cent of the world's earthquakes occur.