WASHINGTON - US seismologists issued an Indian Ocean-wide tsunami watch following a massive earthquake off the coast of Sumatra Wednesday but said it appeared to have only generated small waves.

The 8.6-magnitude earthquake was followed by a massive aftershock, measuring 8.2, off the Indonesian island, raising a new tsunami threat.

Victor Sardina, a geophysicist with the US Pacific Tsunami Warning Center in Hawaii, told AFP the first tsunami was “not anywhere near” as large as the devastating waves that struck Asia in 2004 and Japan last year.

He said the tsunami measured a mere 35cm near Padang, Indonesia, but could swell to as high as a meter 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 Pacific Tsunami Warning Center in a bulletin after the quake added that “sea level readings indicate a tsunami was generated. It may already have been destructive along some coasts.”

“Based on these data the threat continues for all coastal areas of the Indian Ocean,” the Hawaii based center said.

The first quake struck at 2:38 pm (0838 GMT, 4.38pm Singapore time) at a depth of 33km with its epicenter some 435km from Banda Aceh.

The US Geological Survey said the aftershock struck at 10:43 GMT, 615km 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 9.2, 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.