Rescue mission under way for researchers taken hostage in Papua New Guinea

A mission is under way to rescue an Australian professor and three local researchers being held hostage by an armed group in Papua New Guinea (PNG).

Police chief David Manning said security forces would use "whatever means necessary" up to and including lethal force to free the group.

The team were on a field study trip in remote highlands when they were captured, on an unspecified date.

A ransom has been demanded for their release, local officials say.

The Australian Broadcasting Corporation has reported that a larger group was initially kidnapped and some local guides have since been released.

The four people who remain captive are the professor, an archaeologist from an Australian university, and three university students from PNG.

The police chief said overnight the kidnappers were "opportunists" who had "obviously not thought this situation through".

"We are offering the abductors a way out," he said.

"They can release their captives and they will be treated fairly through the criminal justice system, but failure to comply and resisting arrest could cost these criminals their lives."