A strong 6.1-magnitude earthquake struck in the Bismarck Sea near Papua New Guinea on Thursday but it was unlikely to generate a tsunami, a spokesman for the country's Geological Survey said.

"It was at a fairly shallow depth but probably not big enough to cause a tsunami," assistant director Chris McKee said from the capital Port Moresby.

There were no immediate reports of casualties or damage, he said, adding that "it was a long way from any inhabited area."

The US Geological Survey said the earthquake struck at a depth of 10 kilometres about 165 kilometres south southwest of Lorengau, Manus Island and 680 kilometres north of Port Moresby.

The New Britain region of the Pacific island nation is home to many active volcanoes and sits on the Pacific "Ring of Fire" where several tectonic plates collide.

The region is frequently rocked by earthquakes.