An earthquake of 7.5 magnitude has hit near India's Nicobar Islands, in the Indian Ocean.

The US Geological Survey said it occurred about 150km (95 miles) west of the Nicobar Islands and 440km from Sumatra, Indonesia.

The Pacific Tsunami Warning Center initially issued a warning for the entire Indian Ocean region.

The agency later downgraded the warning to India only, before cancelling the alert altogether.

"Sea level readings indicate that a significant tsunami was not generated," the Hawaii-based centre said.

No damage was reported in the Andaman and Nicobar islands, although a total blackout sparked panic in the capital, Port Blair, witnesses said.

More than 350,000 people live on nearly 600 islands in the remote archipelago.

In 2004, a powerful earthquake off Sumatra's coast triggered a tsunami that killed more than 220,000 people around the rim of the Indian Ocean.