Jakarta  - An undersea earthquake registering 6.4 on the Richter scale struck the western coast of the Indonesian island of Sumatra early Thursday, but there were no immediate reports of injury or damage, the Indonesian seismologist agency said.

The quake struck at 3:01 am (2001 GMT Wednesday) and was centred in the Indian Ocean, 32 kilometres south-west of Mentawai Islands, off the western coast of Sumatra, Indonesia's National Meteorology and Geophysics Agency said in a statement.

It occurred 50 kilometres beneath the seabed, but the statement did not say if the quake caused a tsunami threat.

Indonesia, the world's largest archipelago nation, sits along the Pacific Ocean's Ring of Fire, where the meeting of continental plates causes high volcanic and seismic activity.