WASHINGTON (AP) _ A strong earthquake struck the Indonesian island of Sumatra early Sunday, but there was no immediate report on damage or injuries, the U.S. Geological Survey said Saturday.

The tremor, with a preliminary magnitude of 6.6 on the Richter scale, occurred at 2:22 a.m. Sunday local time, or 3:22 p.m. EDT Saturday, said USGS spokesman Don Finley.

It was the strongest earthquake in that area since a magnitude 7.4 quake struck Nov. 17, 1984, Finley said.

''We don't have any reports on casualties or damage,'' Finley said, noting that the ''whole island of Sumatra is pretty heavily populated.''

USGS monitors in Golden, Colo., indicated the tremor was centered in northern Sumatra, just south of Medan, about 800 miles northwest of the Indonesian capital of Jakarta and about 400 miles west of Singapore, he said.

The Richter scale is a gauge of the energy released by an earthquake, as measured by the ground motion recorded on a seismograph.

Every increase of one number, for example from magnitude 5.5 to magnitude 6.5, means that the ground motion is 10 times greater.

A quake of magnitude 6 can cause severe damage, while a tremor of magnitude 7 is considered a major earthquake, capable of widespread, heavy damage.