A massive and shallow earthquake struck on Wednesday off the west coast of the Indonesian island of Sumatra, a region devastated by the 2004 Indian Ocean quake and tsunami, and there were early reports of deaths.

A tsunami alert was called off, TV said. Neighbouring Australia issued a tsunami watch for parts of its western coast and then cancelled it.

"There are some who have died," said Heronimus Guru, the deputy head of operations with the National Search and Rescue Agency. He did not know how many, but any rescue operation will be hampered by the dark, which falls early in the tropical archipelago.

There were no immediate reports of damage, but the shallower a quake, the more dangerous it is. The US Geological Survey originally put the magnitude at 8.2, and then 8.1, before lowering it to 7.9.

The epicentre was 808 km (502 miles) southwest of Padang, USGS said. It was 10 km (six miles) deep.

"So far there have been no reports (of damage)," Andi Eka Sakya, an official of the National Meteorological Agency, told TVOne. "In Bengkulu (on the southwestern coast of Sumatra) they didn't feel it at all."

President Joko Widodo was staying overnight at a hotel in Medan in North Sumatra and was safe, palace officials said. A Medan resident said he did not feel the quake.

Erwin, a resident of Mentawai, a chain of islands off Sumatra, told Metro TV: "I am at the beach currently looking to see any tsunami sign with my flashlight. There's nothing. A few minutes have passed but nothing, but many people have already evacuated to higher places."

Kompas TV said patients at hospitals in Padang were being evacuated. A TVOne reporter said Padang residents were panicking and there were heavy traffic jams.

Indonesia, especially Aceh, was badly hit by the Indian Ocean tsunami in 2004.

A 9.15-magnitude quake opened a fault line deep beneath the ocean on Dec. 26, 2004, triggering a wave as high as 17.4 meters (57 feet) that crashed ashore in more than a dozen countries to wipe some communities off the map in seconds.

The disaster killed 126,741 people in Aceh alone.

Indonesia straddles the so-called "Pacific Ring of Fire", a highly seismically active zone, where different plates on the earth's crust meet and create a large number of earthquakes and volcanoes. - Reuters