An earthquake with an initial magnitude of 8.9 has struck under the sea off Indonesia's northern Aceh province.
The quake triggered a tsunami warning across the Indian Ocean region.
The Pacific Tsunami Warning Center said it was not yet known whether a tsunami had been generated, but advised authorities to "take appropriate action".
The region is regularly hit by earthquakes. The Indian Ocean tsunami of 2004 killed 170,000 people in Aceh.
The US Geological Survey, which documents quakes worldwide, said the Aceh quake was centred 33km (20 miles) under the sea about 495km from Banda Aceh, the provincial capital.
The tsunami centre's warning said quakes of such a magnitude "have the potential to generate a widespread destructive tsunami that can affect coastlines across the entire Indian Ocean basin".
Reuters reports that people in the region were making their way to higher ground.(BBC)
The Meteorology, Climatology and Geophysics Agency (BMKG) issued a tsunami tweeted for the Aceh, West Sumatra, North Sumatra, Lampung and Bengkulu on Wednesday.
The National Oceanic and Atmospheric Administration issued a tsunami warning for the following 27 countries: Indonesia, India, Sri Lanka, Australia, Burma, Thailand, Maldives, United Kingdom, Malaysia, Reunion, Seychelles, Pakistan, Somalia, Oman, Madagascar, Iran, United Arab Emirates, Yemen, Comores, Bangladesh, Tanzania, Mozambique, Kenya, Crozet Islands, Kerguelen Islands, South Africa and Singapore. (Jakarta Globe)