JAKARTA, Indonesia -- A strong undersea earthquake has rocked parts of Indonesia's main Java island, but there were no immediate reports of damage or injuries.

The U.S. Geological Survey says the 6.1-magnitude quake was centered 41 kilometers (25 miles) south of Kroya, in the southern coast of Central Java, with a depth of 83 kilometers (51 miles) under the surface.

Indonesia's Meteorology and Geophysics Agency puts the preliminary magnitude of the Saturday's earthquake at 6.5 but said it would not trigger tsunami.

The world's largest archipelago, Indonesia is prone to seismic upheaval due to its location on the so-called Pacific "Ring of Fire."

A monster temblor off Indonesia's Aceh shores in 2004 triggered a tsunami that killed 230,000 people in a dozen countries. Most of the deaths were in Aceh.(AP)