Guerrilla fighters in Papua's central highlands, who want to free Papua from Indonesia, after he landed a commercial plane in the mountainous area of Nduga.
In a new video released by the group on Friday, a visibly emaciated Mr Mehrtens holds the banned Morning Star flag, a symbol of West Papuan independence, and is surrounded by Papuan fighters brandishing what one analyst said were assault rifles manufactured in Indonesia.
Mr Mehrtens is seen talking to the camera, saying the separatists want countries other than Indonesia to engage in dialogue on Papuan independence.
"If it does not happen within two months then they say they will shoot me," Mr Mehrtens says in the video, which was shared by Papuan rebel spokesperson Sebby Sambom, and verified by Deka Anwar, an analyst at the Jakarta-based Institute for Policy Analysis of Conflict (IPAC).
How have New Zealand and Indonesia responded?
A spokesperson for New Zealand's Ministry of Foreign Affairs said in an e-mail to Reuters on Saturday that they were aware of the photos and videos circulating.
"We're doing everything we can to secure a peaceful resolution and Mr Mehrtens' safe release," the spokesperson added.
Meanwhile, Indonesia's military spokesman Julius Widjojono said on Saturday that the military will continue to carry out 'measurable actions' in accordance with standard operating procedure.
The Indonesian foreign ministry did not respond to requests for comment.
Indonesian authorities have previously said they are prioritising peaceful negotiations to secure the release of the Susi Air pilot, but have struggled to access the isolated and rugged highland terrain.
An increasingly deadly battle for independence
A low-level but increasingly deadly battle for independence has been waged in resource-rich Papua ever since it was brought under Indonesian control in a vote overseen by the United Nations in 1969.
The conflict has escalated significantly since 2018, with pro-independence fighters mounting deadlier and more frequent attacks, largely because they have managed to procure more sophisticated weapons.
Rumianus Wandikbo of the West Papua National Liberation Army - the armed wing of the Free Papua Movement - called on countries like New Zealand, Australia and Western nations to kickstart talks with Indonesia and the separatists.
"We do not ask for money...We really demand our rights for sovereignty," he said in a separate video.
