A massive earthquake struck off Indonesia's Sumatra island on Wednesday, US and Indonesian monitors reported, prompting a Indian Ocean-wide tsunami alert.
The quake's magnitude was 8.7, according to a revised measurement from the US Geological Survey, and it struck 431 kilometres (268 miles) off the Indonesian city of Banda Aceh.
The city was near the epicentre of a devastating 9.1-magnitude quake in 2004 that triggered a tsunami across the Indian Ocean, which killed 220,000 people, including 170,000 in Aceh.
The latest tremor was felt as far afield as Thailand and southern India. Residents in Banda Aceh reported the ground shaking violently.
"People are in a panic, and there are traffic jams everywhere in Banda Aceh," local Metro TV reported.
"There is potential for a tsunami to hit five provinces in the region -- Aceh, North Sumatra, West Sumatra, Bengkulu and Lampung," said Marzuki, an analyst at Indonesia's Geophysics and Meteorology agency.
US monitors issued an Indian Ocean-wide tsunami watch but said it was not yet certain a giant wave had been generated.
"Earthquakes of this size have the potential to generate a widespread destructive tsunami that can affect coastlines across the entire Indian Ocean basin," the US Pacific Tsunami Warning Center said.