US seismologists have cancelled its warning of a possible tsunami following a massive earthquake and aftershock off the coast of Sumatra this morning.

"Sea level readings now indicate that the threat has diminished for most areas, therefore the tsunami watch issued by this center is now cancelled," said the US Pacific Tsunami Warning Center in Hawaii, which monitored currents in the Indian Ocean following the 8.6 magnitude quake and aftershock.

The quake was followed by a massive aftershock, measuring 8.2, off the Indonesian island.

But Victor Sardina, a geophysicist with the Hawaii warning center, told AFP the tsunami was "not anywhere near" as large as those that devastated southeast Asia in 2004 and Japan last year.

Sardina told AFP that the the tsunami measured a mere 35 centimeters (14 inches) near Padang, Indonesia, but could swell to as high as a meter (3 feet 4 inches) near Sri Lanka, adding that US scientists were still carefully monitoring the situation.

Earlier, the center said "earthquakes of this size have the potential to generate a widespread destructive tsunami."

The first quake struck at 2:38 pm (0838 GMT) at a depth of 33 kilometers (14.2 miles) with its epicenter some 435 kilometers (270 miles) from Banda Aceh.

The US Geological Survey said the aftershock struck at 10:43 GMT, 615 kilometers (382 miles) from Banda Aceh.

The area was one of the hardest-hit from the December 26, 2004 quake and subsequent tsunami that wrought devastation across the Indian Ocean and killed some 220,000 people.

The earthquake that caused the 2004 tsunami had a magnitude of 9.1, but also a much stronger vertical component, whereas Wednesday's earthquake was more horizontally directed, Sardina said.

Last year, a 9.0-magnitude earthquake caused a tsunami off northeastern Japan, killing some 19,000 people and sparking a nuclear disaster after an atomic power plant was swamped by the wave.