A magnitude 6.3 earthquake struck off the coast of the Pacific island nation of Vanuatu early Wednesday, the US Geological Survey said, but no tsunami warning was issued.

The quake had a depth of 37 kilometres (23 miles) and struck about 44 kilometres southwest of the capital Port Vila at 1:33 am (1433 GMT), said the USGS, which initially reported a magnitude of 6.5.

The Pacific Tsunami Warning Centre said there was no tsunami threat from the quake, and there were no immediate reports of damage.

Vanuatu's Meteorology and Geohazards Department did not issue a tsunami advisory, according to its website.

Earthquakes are common in Vanuatu, a low-lying archipelago of 320,000 people that straddles the seismic Ring of Fire. 

The Ring of Fire is an arc of intense tectonic activity that stretches through Southeast Asia and across the Pacific basin.

Vanuatu is ranked as one of the countries most susceptible to natural disasters such as earthquakes, storm damage, flooding and tsunamis, according to the annual World Risk Report.