Updated at 9:22 a.m. ET: A 8.6-magnitude earthquake and powerful aftershocks struck off Indonesia on Wednesday, sending people scurrying from shaking buildings in several countries and raising fears of a disastrous tsunami.

Tsunami alerts were issued across the entire Indian Ocean, although they were later lifted.

The U.S. Geological Survey said that the initial quake happened about 14 miles beneath the ocean floor and 270 miles from Banda Aceh, the provincial capital of Aceh, at 2:38 p.m. local time (4:38 a.m. ET).

Reuters reported that the tremors were felt in Thailand, Singapore and southern India.

The quake struck in a similar location to the 9.1-magnitude tremor on Dec. 26, 2004, that triggered a tsunami in the Indian Ocean, killing almost 230,000 people.

An 8.2-magnitude aftershock hit at 6:43 a.m. ET, the USGS said. Fresh tsunami alerts were issued as a result. Several other smaller temblors, of between 5.1 and 5.7-magnitude, were recorded in the same area between 5:51 a.m. ET and 8:10 a.m. ET.

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'The threat has diminished'
Individual countries, including Kenya, issued tsunami warnings for their Indian Ocean coastlines.

The Hawaii-based Pacific Tsunami Warning Center issued a tsunami watch -- an alert category one level below a warning -- for the entire Indian Ocean area but later canceled it.

"Level readings now indicate that the threat has diminished or is over for most areas,'' it said.  

It posted a series of estimated arrival times for tsunami waves for potentially affected areas, saying that if no major waves had happened two hours after those times then “local authorities can assume the threat is passed.”

“As local conditions can cause a wide variation in tsunami wave action, the all-clear determination must be made by local authorities,” the warning center said.

Small tsunami waves of around three feet in height hit the western coast of Sumatra island, Reuters reported, though Indonesia’s disaster agency said it was still assessing whether there were any deaths or damage.

'Remain alert'
Indonesia's Geophysics Agency also said it had detected a rise in sea level of up to 2 feet 7 inches, according to Reuters.Tsunami waves are relatively small in the open ocean, but can quickly build up as they near shore or are channeled into inlets.

National Disaster Mitigation Agency (BNPB) spokesman Sutopo Purwo Nugroho told The Jakarta Post that the large aftershock "might potentially trigger a tsunami. Residents must remain alert."

A witness on Indonesia's Simeulue Island, near the epicenter of the quake, said the sea had receded by about 10 yards; water receding is a sign that a tsunami wave is about to arrive.

NBC News reported scenes of panic in Indonesia, with residents and even hospital patients fleeing buildings.

PhotoBlog: Scenes of panic in Banda Aceh

"The quake was felt very strongly. Electricity is down, there's traffic jams to access higher ground. Sirens and Quran recitals from mosques are everywhere," a spokesman for Indonesia's disaster mitigation agency told Reuters.

In Banda Aceh, Fauvan, who like many people in the region has a single name, told NBC News that when the quake struck "the buildings shook for three to four minutes --  it was very scary."

"I immediately left the building and ran towards higher ground. A lot of people did the same. There were a lot of people in the street," she said.

She said she had now returned to the hotel. "It is quite good here now" she said, sensing the worst of the quake and tsunami warning had passed.

Indonesia's disaster mitigation agency is sending a rescue team to Aceh province, and said electricity had been cut to the area.

Thousands evacuated
Several thousand people were evacuated to higher ground from parts of India's Andaman and Nicobar islands as authorities prepared for waves of up to 12 feet or more.

India earlier issued a tsunami alert for its eastern coast, saying waves measuring almost 20 feet high might strike parts of its eastern coast. Hundreds of office workers in the Indian city of Bangalore left their buildings.

Chaideer Mahyuddin / AFP - Getty Images

Acehnese women hug each other and pray shortly after a powerful earthquake hit the western coast of Sumatra, Indonesia.

Reports on Twitter and elsewhere said Wednesday's first quake was also felt in Singapore, Sri Lanka, Thailand and Malaysia. High-rise apartments and offices on Malaysia's west coast shook for at least a minute.

Evacuation orders were issued for Thailand's southern island of Phuket and another southern province, Phangnga. "The province has turned on the warning sirens and asked people all over Phuket island to move to a safe place," an official from the Phuket Disaster Prevention and Mitigation Center told Reuters by telephone.

The country's National Disaster Prevention Center told NBC News that Phuket airport has been temporarily closed and flights diverted elsewhere.

Previous Top 10 deadliest earthquakes in history

Nadine Hills, a British honeymooner on vacation near Ao Nang, Thailand, told msnbc.com that hotel guests had been evacuated to a local school. "There's a lot of people here and a massive storm flashing and rumbling over head. The police just arrived with water and to tell us we're safe for now. Just going to have to sit tight and see."

Simon Boxall, a U.K. oceanographer, told Sky News that the danger would not necessarily be over if the first quake did not produce a tsunami.

"The initial earthquake may not cause a tsunami … [but] there's no reason why an aftershock, which could still reach up to 8 in magnitude, cannot still cause a tsunami," he added, speaking before the first aftershock hit.

Boxall told Sky that not all offshore quakes produced tsunamis and issuing evacuation orders every time there was one could start to "get very messy."

NBC News reported that quake has been rated as a '5' on the USGS MMI scale, which measures the physical intensity of an earthquake as felt on the ground. A strength 5 quake is defined as: "Felt inside by most, may not be felt by some outside in non-favorable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house."

British Prime Minister David Cameron, who is on a pre-arranged official visit to the Indonesian capital, Jakarta, announced that Britain "stands ready to help if required."

Area prone to volcanic and seismic activity
Indonesia straddles a series of fault lines that makes the vast island nation prone to volcanic and seismic activity.

According to the USGS, the 2004 quake struck about 155 south-southeast of Banda Aceh at a depth of 18.6 miles. Some 227,898 people were killed or missing presumed dead and about 1.7 million were forced out of their homes after the  tsunami affected 14 countries in Asia and East Africa.

Indonesia's explosive geology explained

"This is the third largest earthquake in the world since 1900 and is the largest since the 1964 Prince William Sound, Alaska earthquake," the USGS said in its summary about the 2004 earthquake.

"The tsunami caused more casualties than any other in recorded history and was recorded nearly world-wide on tide gauges in the Indian, Pacific and Atlantic Oceans," it added.

The most severe earthquake since 1900 was of 9.5 magnitude and struck Santiago and Concepcion in Chile on May 22, 1960, triggering tidal waves and volcanic eruptions. Some 5,000 people were killed and two million made homeless.

Reuters contributed to this report.

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