India on Wednesday downgraded tsunami alerts across the country that had been issued after an 8.6-magnitude earthquake struck off the coast of Sumatra.

"The expected period of significant tsunami waves is now over for all threatened Indian coastal areas," said a bulletin from the national tsunami early warning centre in the southern city of Hyderabad.

An official at the centre, S. A Kishore, said the highest tsunami warning level issued for the Andaman and Nicobar islands in the Bay of Bengal had been downgraded from red to orange.

Earlier, a few thousand people were evacuated to higher ground from parts of India's Andaman and Nicobar islands on Wednesday following earthquakes in Indonesia, as the islands prepared for tsunami waves up to 3.9 metres high, officials said.

"There could be high waves of 1.5 metres at Port Blair and 3.9 metres at Campbell Bay," said Prabhakar Rao, the official in charge of the disaster control room at Port Blair, the main town on the islands.

Small waves about half-a-metre high and within normal tide limits had already washed into the Campbell Bay area on the Great Nicobar island, the official said.

"We have evacuated people from low lying areas of Campbell Bay islands and we have also alerted them as there is a report that high waves may hit Indira Point area," south Andaman region police superintendent S.B.S. Tyagi said.

India on Wednesday added the coastal states of Kerala, Orissa, Goa and West Bengal to a tsunami warning along with Andhra Pradesh and Tamil Nadu.

A more urgent "red alert" warning was earlier issued for the Andaman and Nicobar Islands, located in the Indian Ocean, after a massive earthquake off the coast of Sumatra.

"We have evacuated a few hundred people. There is no report of any damage so far," Shakti Sinha, chief secretary of Andaman and Nicobar islands, told AFP by telephone.

Video: Click here to watch 10 biggest recorded earthquakes

A tsunami watch was issued for 22 countries across the Indian Ocean after a large earthquake hit waters off Indonesia on Wednesday, triggering widespread panic as residents along coastlines fled to high ground in cars and on the backs of motorcycles.

Earlier, India issued a tsunami warning for the Andaman and Nicobar Islands, located in the Indian Ocean, after a massive earthquake off the coast of Sumatra.

Click here to read the largest earthquakes since 1900

The Indian National Centre for Ocean Information Services issued a red high-level warning for the islands, and also put out lower alerts for the coasts of Andhra Pradesh and Tamil Nadu states in the southeast of the country.

The institute, based in the southern city of Hyderabad, is home of the Indian Tsunami Early Warning Centre.

In the 2004 tsunami, the Indian government said 454 people died in the islands and listed 3,073 as missing.

More than 350,000 people live in the Andaman and Nicobar Islands.

Tsunami risk seems low, says seismologist

The risk of a tsunami being generated by the giant earthquake off Sumatra on Wednesday is low, an expert with the British Geological Survey (BGS) said.

The earthquake's movement was horizontal, not vertical, and caused no apparent movement of the sea floor, which is what triggers tsunamis, seismologist Susanne Sargeant told AFP in Paris by phone.

"We've had two blocks rubbing together, it's called a strike-slip earthquake.

"That means there hasn't been any displacement of the sea floor. Although an earthquake of this magnitude has the potential to cause a large tsunami, the fact that we haven't seen any drop of the sea floor, which is what generates the wave, it looks like the possibility of a tsunami being generated is low."

The US Geological Survey (USGS) said an 8.6-magnitude earthquake struck off the coast of Sumatra at 2:38 p.m. (0838 GMT) at a depth of 33 kilometers (14.2 miles).

Tsunami alerts or evacuation orders were issued for parts of Malaysia, India, Indonesia, Sri Lanka and Thailand.

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