A strong 6.5-magnitude earthquake struck beneath the sea off Papua New Guinea on Monday, US seismologists said, prompting a local tsunami alert.

There were no immediate reports of casualties or damage from the quake, which hit at 4:37 am (1837 GMT Sunday) at a depth of 43 kilometres (27 miles) some 120 kilometres west of the island of Bougainville in Papua New Guinea, according to the United States Geological Survey.

"Earthquakes of this size sometimes generate local tsunamis that can be destructive along coasts located within 100 km of the earthquake epicentre," the Pacific Tsunami Warning Center, based in Hawaii, said in a statement.

There was no risk of a destructive widespread tsunami from the quake, which was also 248 kilometres from Chirovanga in the Solomon Islands, the centre said.

Papua New Guinea, which is mired in poverty despite rich mineral deposits, sits on the so-called "Pacific Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.

A giant tsunami in 1997, caused by an undersea earthquake or a landslide, killed more than 3,500 people near Aitapi, on the northwest coast of mainland Papua New Guinea.

In 2007 a tsunami following an 8.1-magnitude earthquake killed at least 52 people in the Solomon Islands and left thousands homeless.