A strong 6.5-magnitude earthquake struck the Western Solomons, beneath the sea off Papua New Guinea, prompting a local tsunami alert, US seismologists say.

There were no immediate reports of casualties or damage from the quake, which hit at 4.20am this morning, at a depth of 43 kilometres some 248 kilometres from Chirovanga in the Solomon Islands.

The center was said to be 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 epicenter," the Pacific Tsunami Warning Center, based in Hawaii, said in a statement.

Solomon Islands, along with Papua New Guinea, sits on the so-called Pacific Ring of Fire, a hot spot 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 3500 people near Aitapi, on the north-west 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.