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

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

Residents 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 of the coast of Papua New Guinea coast, near the town of Rabaul, in the country's northeast.

A tsunami warning was issued soon after the PNG quake, though the Pacific Tsunami Warning Centre said no destructive, Pacific-wide tsunami was expected.