A 6.5-magnitude earthquake hit off the coast of the South Pacific island of Vanuatu on Sunday, the Hong Kong Observatory said, but there were no reports of a tsunami warning.

The quake struck at 1640 GMT around 81 kilometres southwest of the capital Port Vila.

The shallow quake, which had a depth of four kilometres, was measured by the US Geological Survey as having a magnitude of 6.0 on the Richter scale.

There were no immediate reports of casualties or damage and the Hawaii-based Pacific Tsunami Warning Center had yet to issue any information bulletin or alert.

Vanuatu lies on the so-called "Pacific Ring of Fire", a zone of frequent seismic activity caused by friction between shifting tectonic plates.