The US Geological Survey said that the quake's epicenter was some 20 miles beneath the ocean floor around 308 miles from the provincial capital of Banda Aceh.
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Indonesia's Disaster Mitigation Agency said that an aftershock of 6.5 magnitude was felt in Aceh an hour later. A second, 8.1-magnitude aftershock hit the region at 2 pm.
An official at Indonesia's Meteorology and Geophysics Agency said tsunami warnings have been issued across the Indian Ocean.
Experts predict ocean turbulence following the quake, and warned that a subsequent tsunami may see waves of 19 feet and over.
The earthquake was felt in 28 countries including India, Singapore, Malaysia and Nepal. Tremors were reported across India including Kolkata, Chennai, Bangalore, Bihar and Guwahati.
(Photo: Google Maps)
The tsunami warnings include Indonesia, India, Sri Lanka, Australia, Myanmar, Thailand, and the Maldives; as well as Malaysia, Pakistan, Somalia, Oman, Iran, Bangladesh, Kenya, South Africa and Singapore.
Thailand's national Disaster Prevention Center issued tsunami warnings for six southern provinces along the Anadaman coast. People in Phuket, Krabi, Ranong, Phangnga, Trang and Satun were ordered to move to higher ground because of a possible tsunami, a Thai official said.
"The province has turned on the arning sirens and asked people all over Phuket island to move to a safe place, we warn people in every district to evacuate immediately because there is a possibility of a tsunami," the official added.
Sutopo, spokesman for the center said that "the electricity is down, and there are traffic jams to access higher ground. Sirens and Koran recitals from mosques are everywhere." The Phuket Airport has been closed.
President Susilo Bambang Yudhoyono told reporters: "There is no tsunami reported so far, but we remain vigilant. Our warning system is working well, and I have ordered the national relief team to fly immediately to Aceh to ensure the situation is under control and to take any necessary action," he said.
"Thanks God, from what I heard there is neither casualties reported nor major damage in Banda Aceh or other places," he added.
Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.
A giant 9.1-magnitude quake off the country on December. 26, 2004, triggered a tsunami in the Indian Ocean that killed 230,000 people, nearly three quarter of them in Aceh.
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Reuters and AP contributed to this report