A massive earthquake has struck off Indonesia's Sumatra island, US and Indonesian monitors report, prompting a tsunami alert across the Indian Ocean.
And a fresh tsunami warning has been issued after a strong 8.2-magnitude aftershock rattled the same area.
The US Geological Survey (USGS) said the aftershock struck at 2043 AEST 615 kilometres from Banda Aceh.
This follows expert advice that the first tremor had been horizontal, rather than vertical, and that a tsunami was unlikely.
Southeast Asian nations had earlier issued tsunami alerts and urged people to move to safety away from coastlines after a massive 8.6 magnitude earthquake struck off the coast of Indonesia's Sumatra on Wednesday.
Sri Lanka, India, Thailand and Malaysia issued evacuation instructions as US monitors issued an Indian Ocean-wide tsunami watch after the quake, which according to the USGS struck off the coast of Sumatra at 6.38pm AEST at a depth of 22 kilometres. USGS had initially reported it as an 8.9-magnitude quake.
German experts who helped install the tsunami warning system off Indonesia said the quake was horizontal, rather than vertical, making a big tsunami less likely.
"There wasn't much vertical movement," said Rainer Kind, of the German Geo-Research Centre (GFZ), which helped to install the system after a 2004 tsunami killed thousands in the same area.
Based on data from the system, "it's very likely that it generated a tsunami," Kind said from the GFZ headquarters at Potsdam near Berlin.
But the size of the wave was "very difficult to predict, because we don't know the topography of the seabed", he said. Larger tsunamis are usually created by quakes with vertical movement, or by landslides into the sea.
An analyst at Indonesia's Geophysics and Meteorology agency said five of the nation's provinces - Aceh, North Sumatra, West Sumatra, Bengkulu and Lampung - were at potential risk.
Sri Lanka and India issued tsunami warnings while Thailand urged people on the Andaman coast, a popular tourist destination, to move to safety. Malaysia ordered a coastal evacuation.
A Sri Lanka government statement said potential waves could hit the island's eastern coast by about 8.40pm AEST and urged an orderly evacuation of the coastal strip.
"There is a strong possibility of a tsunami hitting the island after the earthquake in Indonesia," meteorological department deputy director M. D. Dayananda said.
He said the quake in Indonesia was felt in Sri Lanka, which is 1340 kilometres northwest of the quake epicentre.
India issued a tsunami warning for the Andaman and Nicobar Islands, located in the Indian Ocean. The Indian National Centre for Ocean Information Service issued a red high-level warning for the islands, and also put out lower alerts for the coasts of Andhra Pradesh and Tamil Nadu states in the southeast of the country.
Thailand's National Disaster Warning Centre advised people in the area to move to higher places and stay as far away as possible from the sea. The quake swayed buildings in the capital Bangkok.
Australian Bonnie Muddle, vacationing on the Thai resort island of Phuket at the time of the quake, said people were being evacuated from popular tourist areas including Krabi and Phang Nga Bay.
"Everyone is getting a little concerned over here," she said.
On Wednesday Japan's Meteorological Agency said that there was no risk of a tsunami affecting Japanese coasts.
Geoscience Australia, Canberra's geohazards agency, said there was no risk to Australia from the jolt. Taiwan and New Zealand also said the earthquake posed no threat to them.