The 37-year-old married father-of-one is being held by the West Papua National Liberation Army, a rebel group who have repeatedly threatened to execute him unless the province is granted independence from Indonesia.
Indonesia’s army and police have been sending forces into the rugged mountainous terrain to try to rescue Mehrtens, while New Zealand has been involved in diplomatic discussions with representatives from both sides.
Supporters of Mehrtens had in recent weeks hoped that negotiators were close to securing his release as his captors softened their rhetoric.
But on Friday the group issued a stark warning in response to Indonesia’s escalating military action: “We will carry this pilot around on the battlefield until he dies together with us.”
The force said they had recently moved Mehrtens to a village under strict guard as negotiations progressed.
“We thought that Indonesia had good intentions … however Jakarta carried out brutal bombings,” Sebby Samboom, the group’s spokesman said.
Noticeably more solemn than in previous proof-of-life videos, a heavily bearded Mehrtens could be seen with his hands behind his back while fighters beside him brandished guns, spears, bows and arrows.
He described the military action as life-threatening as he requested that foreign countries talk to Indonesia and request that they stop the air strikes.
Samboom said helicopters and drones had been deployed in Kwiyawagi, an area that had previously been deemed a safe zone for civilians.
A Papuan child aged 13 was one of two civilians killed during the intensifying fighting this week, Samboom said.
Photographs produced by the group appeared to show several dead and wounded Papuans. They could not be independently verified.