An underwater magnitude 8.7 earthquake has struck in the Indian ocean off the coast of Indonesia.
Seismologists from the US Geological Survey (USGS) said the earthquake hit near Aceh province on the island of Sumatra.
Tsunami warnings were issued in Indonesia and India and people rushed to high ground in Aceh province, but there has been no major destruction yet reported.
There has also been no low tide seen on the Indonesian coast, according to media reports, which would have been expected if a tidal wave was about to hit.
Prime Minister David Cameron is currently in Indonesia on a tour of Asia.
At a press conference with the Indonesian PM in Jakarta, focused on trade and business, Cameron said that Britain would be ready to help Indonesia "at this time of worry".
"Earthquakes of this size have the potential to generate a widespread destructive tsunami that can affect coastlines across the entire Indian Ocean basin," the Pacific Tsunami Warning Center said.
"However it is not known that a tsunami was generated... Authorities in the region should take appropriate action in response to the possibility of a widespread destructive tsunami."
The BBC said that in Jakarta buildings shook for up to five minutes during the quake.
The tremors were said to have been felt as far afield as the eastern Indian coast and Thailand.
The quake was located 33km under the sea, the USGS said. Its epicentre was about 495km from Banda Aceh, the captial of Aceh province.
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Initially the quake was reported as magnitude 8.9 but that was later downgraded.
Indonesia is located on the so-called 'ring of fire', a deadly region of major seismic activity and one of the most active faultlines.
In 2004 an earthquake off the coast of Sumatra resulted in a tsunami which killed around 230,000 people on the coast of the Indian ocean.
That quake was one of the longest ever recorded and was reported to be between magnitude 9.1 and 9.3.