Washington: US monitors issued an Indian Ocean-wide tsunami watch following a massive earthquake off the coast of Sumatra on Wednesday but said it was not yet certain a giant wave had been generated.

"Earthquakes of this size have the potential to generate a widespread destructive tsunami that can affect coastlines across the entire Indian Ocean basin," the US Pacific Tsunami Warning Centre said.

The US Geological Survey said an 8.7-magnitude earthquake struck off the coast of Sumatra at 1408 IST at a depth of 33 kilometers. USGS had initially reported it as an 8.9-magnitude quake.

The tsunami warning center said it had not yet ascertained whether a tidal wave had been generated but that one was likely given the strength of the temblor, and advised local authorities to take "appropriate action."

On December 26, 2004 a 9.2-magnitude earthquake off Sumatra generated a catastrophic tsunami that wrought devastation across southern Asia, killing an estimated 220,000 people.

Last year, a 9.0-magnitude earthquake caused a tsunami and nuclear disaster in Japan, killing some 19,000 people.