SYDNEY (AFP) - A powerful earthquake with a magnitude of 7.5 struck off Papua New Guinea late Saturday night, the United States Geological Survey (USGS) said, with a tsunami warning issued.

The quake hit 75km south-west of the town of Panguna on Bougainville island, USGS said, and came after a 6.6-magnitude tremor earlier on Saturday.

A tsunami warning was issued for Papua New Guinea and the Solomon Islands, the Hawaii-based Pacific Tsunami Warning Centre said, adding that it was "not known that a tsunami was generated".

The quake struck at a depth of 30km just before 11.30pm, USGS said. It reported another 5.6-magnitude quake had happened around 20 minutes later in the same area at a depth of 69km.

A 6.6-magnitude quake occurred in the same area on Saturday morning, USGS reported earlier.

Quakes of such magnitude are common in Papua New Guinea, which sits on the so-called Pacific "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.