A strong undersea earthquake struck off Indonesia's Sumatra island on Saturday, although there was no tsunami warning issued, the country's meteorology agency said.

There were no immediate reports of damage or casualties, but an agency official said the quake was felt strongly in several districts in South Sumatra.

The magnitude 6.7 quake was at a depth of 23 km (14.3 miles), with the epicentre 142 km southwest of Bengkulu, the agency said in a telephone text message.

A second quake measuring 6.3 struck the same area later, the agency said.

The U.S. Geological Survey put the first quake at a magnitude 6.8 and at a shallower depth of 4 km.

Indonesia suffers frequent earthquakes lying on an area of intense seismic activity where a number of tectonic plates collide.

The sprawling archipelago of more than 17,000 islands was hit by a devastating tsunami four years ago that left an estimated 170,000 people dead or missing in Aceh province on the northern tip of Sumatra.

Last week Indonesia launched a new hi-tech system aimed at detecting a potential tsunami and providing faster alerts, though experts say large parts of the country are still not covered and the system will not be fully operational until 2010.