An Australian archaeologist is one of five people taken hostage in a remote area of Papua New Guinea by ransom-demanding armed gunmen.
The academic was reportedly held at gunpoint by 20 armed men, according to a PNG police incident report.
Local guides who were also taken hostage have been released.
PNG Prime Minister James Marape said the five hostages were alive and negotiations were under way, with police and military on stand-by to assist.
"Do yourself and do your country a favour by releasing the people concerned," he said, addressing the captors via a media conference in Port Moresby on Monday.
The gunmen have demanded $3.5 million kina ($A1.4 million) be paid.
Mr Marape reassured the families of the hostages that PNG officials had been "at work".
"Contact has been made with people in the bush through secondary sources," he said.
"They have indicated ransom. We do not encourage ransom, but we're treating this very diligently and carefully because life is at risk and life is at stake."
Mr Marape said the government had made "good inroads" in the negotiations through church missionaries and villagers.
"I am positive, I am optimistic, I am prayerful that we get the five out," he said.
The prime minister said the armed men descended into a village and were confronted by police, who discovered they had the researchers with them.
The professor's university declined to comment.
Prime Minister Anthony Albanese told reporters in Perth he would seek a briefing on the matter.
The Department of Foreign Affairs is declining to comment.
The Royal Papua New Guinea Constabulary, the national police force, has been contacted for comment.
