Papua New Guinea’s prime minister has issued a stern warning to kidnappers to release their remaining hostages after one was freed.

Armed men took four people hostage, including a New Zealand national who has been a long-time Australian resident, and are demanding a ransom from the PNG and Australian governments.

James Marape urged the gang to set the remaining hostages free.

“You have no place to hide – all of you, your names, and your face, are being profiled as we speak,” Mr Marape said on Thursday.

“We have over 13 names and pictures of all of you in the mountains.”

The four researchers, including the Australian professor, had been working in the Southern Highlands area.

Local police believe the crime was opportunistic.

Missionaries based in the region have been acting as intermediaries during negotiations.

– AAP