Jakarta, Indonesia (CNN) -- A massive earthquake struck off the coast of the Indonesian island of Sumatra on Wednesday afternoon, triggering a tsunami alert for the Indian Ocean.

The quake struck about 434 kilometers (270 miles) southwest of Banda Aceh, the capital of Indonesia's Aceh province, and had a magnitude of 8.6, the U.S. Geological Survey said. It took place at a depth of 23 kilometers (14 miles).

A second quake, magnitude 8.2, occurred off the west coast of Sumatra about two hours later, the USGS said.

Gary Gibson from the Seismology Research Center in Melbourne, Australia, said the location of the second quake reduces the possibility of a tsunami.

There were no immediate reports of destruction or deaths Wednesday.

Indonesian President Susilo Bambang Yudhoyono said on local television that there were no reports of casualties or damage in Aceh.

The Pacific Tsunami Warning Center issued a tsunami watch for the entire Indian Ocean, and the Indonesian Meteorology, Climatology and Geophysics Agency said it had put up a tsunami warning as well.

Sea level readings indicate a tsunami was generated, the warning center said. "It may already have been destructive along some coasts."

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The center also said that "when no major waves have occurred for at least two hours after the estimated arrival time or damaging waves have not occurred for at least two hours, then local authorities can assume the threat is passed." The center posted approximate arrival times for waves in different parts of the region, which were predicted for different times in different cities throughout the day.

Waves were reported at 1-meter (3.3-foot) amplitude offshore in Meulaboh, Indonesia, but in other cities they were reported at about a foot or less, according to the warning center.

British Prime Minister David Cameron announced that Britain "stands ready to help if required."

The first quake took place at 2:38 p.m. local time (4:38 a.m. ET).

Officials called on coastal residents in low-lying areas in the region to seek higher ground.

The power went out in Banda Aceh and residents were moving to higher ground, said Sutopo Purwo Nugroho, a spokesman for the Indonesian National Disaster Management Agency.

The areas most at risk of a tsunami are coastal areas of Aceh, particularly the island of Simeulue, Prih Harjadi, an official for the Indonesian geophysics agency said on Metro TV.

The earthquake appears to have involved a horizontal movement rather than a vertical movement, so it is less likely that it will generate a tsunami, Gibson said.

He also said that the tremor took place a long way offshore and was therefore unlikely to have caused much damage itself.

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In 2004, a 9.1-magnitude underwater earthquake off the coast of Sumatra, triggering a tsunami that killed more than 200,000 people in 14 countries. The majority of the deaths were in Indonesia, with Aceh bearing the brunt.

That quake took place 250 kilometers (155 miles) south-southeast of Banda Aceh at a depth of 30 kilometers (19 miles).

The tsunami, which washed away entire communities, caused nearly $10 billion in damage and more casualties than any other tsunami in history, according to the United Nations.

Since then, officials have worked to improve warning systems and have carried out drills in the region.

Indonesia is on the so-called Ring of Fire, an arc of fault lines circling the Pacific Basin that is prone to frequent earthquakes and volcanic eruptions.

The earthquake Wednesday comes just over year after a magnitude 9 quake off the northeast coast of Japan caused a devastating tsunami. The death toll from that disaster stands at about 15,850.