A strong underwater earthquake has struck near the Indonesian island of Sumatra, but there are no immediate reports of casualties or damage.

Indonesia's meteorological agency says the quake measured 6.2 on the Richter scale and occurred Thursday hundreds of miles off the coast of Aceh province in northern Sumatra.

The U.S. Geological Survey estimated the strength at 6.0.

Earlier this week, a magnitude 6.2 quake struck off the western coast of Sumatra at a depth of 33 kilometers.

A series of powerful earthquakes and dozens of aftershocks in the same region last month killed at least 23 people and damaged or destroyed thousands of buildings.

The Indonesian archipelago is prone to seismic activity because of its location on an arc of volcanos and fault lines circling the Pacific Basin known as the Ring of Fire.

In December 2004, a tsunami triggered by an underwater earthquake killed an estimated 230,000 people near Indian Ocean coastlines, including more than 160,000 people in Indonesia's Aceh province.

Some information for this report was provided by AFP, AP and Reuters.