BANDA ACEH, Indonesia -- Two strong earthquakes struck in waters off the western coast of the Indonesian island of Sumatra on Wednesday, with an initial 8.6-magnitude shock followed some two hours later by an 8.2-magnitude tremor.

The first quake hit about 2:30pm local time at a depth of 14 miles. The epicenter was located 269 miles southwest of Banda Aceh, triggering an Indian Ocean-wide tsunami warning.

The United States Geological Survey (USGS) said a second major quake measuring 8.2 magnitude hit at about 4:40pm local time at a depth of 10 miles.

Experts said the first tremor was caused by tectonic plates moving horizontally on the sea floor and that the chance of a large-scale tsunami was minimal.

In 2004, a 9.1-magnitude earthquake struck off the western coast of Sumatra, triggering a huge tsunami in the Indian Ocean that killed some 230,000 people. That quake was caused by tectonic plates moving vertically.

Bruce Presgrave from the USGS told Sky News that efforts were underway to determine the nature of Wednesday's second quake and whether it may have disturbed tectonic plates vertically, leading to a higher likelihood of a tsunami.

"We are still looking at this second earthquake. This one is so close in size to the first event that it could have a different mechanism," Presgrave said. "That is something we are looking at right now. It is within a few hundred kilometers of the first event."

Southeast Asian nations issued tsunami alerts immediately following the first quake and urged people to move safely away from coastlines.

The Pacific Tsunami Warning Center reported a tsunami measuring 3.5 feet (one meter) in Meulaboh, Indonesia, in the aftermath of the first quake. A water level increase of about one foot was also reported in Sabang. A four-inch tsunami was reported on the Thai coast, according to a disaster center cited by AFP.

British prime minister David Cameron was on a visit to Jakarta at the time the earthquake struck. Cameron held a news conference in Jakarta with Indonesian president Susilo Bambang Yudhoyono, who told reporters that there were no reports of any casualties caused by the first quake.

Yudhoyono, who spoke before the second major temblor, said that there was no imminent threat of a tsunami in Banda Aceh but added that authorities were "on alert."

An AFP correspondent in Banda Aceh said the first earthquake "began as a mild tremor and then got stronger," adding that the ground shook for about five minutes and telephone connections went down briefly.

"There are people trying to evacuate. Some are praying, and children at a school were panicking as teachers tried to get them out," he said. "There are traffic jams everywhere as people are trying to get away from the coast -- many are on motorcycles."

People living along India's eastern coast were told to move away from the shore, Sky News reported. Tremors were felt in the cities of Kolkata, Chennai and Bangalore.

The Indian National Center for Ocean Information Service was quick to issue a red high-level warning for the Andaman and Nicobar Islands, located in the Indian Ocean. Lower alerts were issued for the coastlines of Andhra Tradesh and Tamil Nadu, in the country's southeast.

Thailand's National Disaster Warning Center advised people on the Andaman coast to move to higher places and stay as far away as possible from the sea.

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