A strong undersea earthquake has struck Indonesia's Mollucas islands, blacking out power in the town of Labuha, but not triggering a tsunami warning, officials at the country's meteorology agency said on Tuesday.

The quake was widely felt in the remote area made up of scattered islands, but there were no immediate reports of damage or casualties, Suhardjono, an official at the agency, said by telephone.

The epicentre of the quake, which measured 6.1 on the Richter scale, was 40 km (25 miles) southeast of Labuha and was followed by a slight weaker aftershock, another official said.

The U.S. Geological Survey said in a bulletin sent by e-mail that the first quake was at a depth of 10 km and struck at 0936 GMT, followed by an aftershock six minutes later. It put the magnitude of the first quake at 6.0 and the aftershock at 5.6.

Indonesia suffers frequent earthquakes given its location in a highly active seismic zone.

In some remote areas, it takes a long time to assess damage given patchy communications in the sprawling archipelago of more than 17,000 islands.