A powerful magnitude 7.5 quake struck off the Pacific island nation of Vanuatu, the US Geological Survey reported.

It said the quake, which struck at 3:55am on Sunday (16:55 GMT on Saturday), was centered 60km southwest of the town of Port-Vila and was quite shallow, at a depth of 40.5km.

The Pacific Tsunami Warning Center said the major quake had not triggered a Pacific-wide tsunami but said: "Earthquakes of this size sometimes generate local tsunamis that can be destructive along coasts located within 100km of the earthquake epicenter."

There was no immediate information of any damage or casualties, but when on land quakes of this magnitude can cause widespread and heavy damage.

The quake was followed by a weaker aftershock of magnitude 5.9 some 20 minutes later.

The Vanuatu archipelago is located on the so-called "Ring of Fire," one of the most seismically active regions in the world. Port Vila is the capital of Vanuatu and has an estimated population of 40,000 people.