JAKARTA - Indonesia issued a tsunami warning on Wednesday after a massive and shallow earthquake struck off the west coast of its island of Sumatra, a region devastated by the 2004 Indian Ocean quake and tsunami.

The warning was issued for West Sumatra, North Sumatra and Aceh after the quake of magnitude 7.9, the National Meteorolgical Agency said, but rescue efforts will be hampered by the darkness that falls early in the tropical archipelago.

The epicentre was 808 km (502 miles) southwest of Padang, the US Geological Survey said. It was 10 km (six miles) deep.

There were no immediate reports of damage or casualties, but the shallower a quake, the more likely it is to cause damage. USGS originally put the magnitude at 8.2, and then 8.1, before lowering it to 7.9.

"So far there have been no reports (of damage) yet," Andi Eka Sakya, an official of the National Meteorological Agency, told TVOne. "In Bengkulu (on southwest 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 didn't feel the quake.

Neighbouring Australia issued a tsunami watch for parts of its western coast.

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.