A 6.9-magnitude earthquake hit off the coast of Papua New Guinea's New Britain island, the U.S. Geological Survey (USGS) said on Thursday, triggering a tsunami warning for surrounding coastlines.

There were no immediate reports of casualties or damage from the shallow quake, which was initially reported at the magnitude of 7.2 close to the coast.

The Hawaii-based Pacific Tsunami Warning Center (PTWC) issued a threat warning for parts of the southwestern Pacific.

Tsunami waves ranging from 30 centimeters to 1 meter above tide level were forecast for parts of Papua New Guinea.

"Government agencies responsible for threatened coastal areas should take action to inform and instruct any coastal populations at risk," the PTWC said in its alert.