A series of strong earthquakes struck off the neighbouring South Pacific Ocean states of Samoa and Tonga on Monday, the US Geological Survey said, just hours after a major tremor rattled Papua New Guinea to the west.

A number of 6.8 magnitude quakes struck south-west of the Samoan capital Apia, in waters between the two island states of Tonga and Samoa.

Residents and police in Samoa told Reuters there were no reports of damage and the Pacific Tsunami Warning Centre said there was no tsunami threat.

The quakes came just hours after a major 7.7 quake struck off the coast of Papua New Guinea, near the town of Rabaul, in the country's north-east.