The separatist Papua rebel group that has kidnapped a New Zealand pilot has threatened to execute him.

In an alarming statement issued on Wednesday morning, West Papua National Liberation Army (TPNPB) spokesman Sebby Sambom said Kiwi pilot Philip Mehrtens was still alive.

In a statement on social media, Mr Sambom said Mr Mehrtens was being held hostage for negotiations with Jakarta.

“If Jakarta is obstinate, then pilot will be executed to death,” Sambom said.

“Later we from the management of the TPNPB-OPM Central Headquarters will monitor.”

Tweet from @IndoPilot

Mr Mehrtens, a 36-year-old married father, was taken hostage late on Tuesday while flying a plane for Indonesian regional airline Susi Air. The small plane was carrying five passengers, one a baby, from Mozes Kilangin Airport in Mimika, in Central Papua, to Paro Airport in Nduga.

It plane apparently landed safely in remote Nduga before being attacked by rebel fighters and set alight.

The fate of the passengers has not been confirmed. Mr Sambom said an earlier statement they had been released before the fire.

“We want to convey that we have taken this pilot hostage and brought it to the TPNPB headquarters, which is far from the airfield area,” he said, warning police and military not to carry out reprisal sweeps or make civilian arrests.

“This pilot is a citizen of New Zealand. TPNPB considers New Zealand, Australia, Indonesia, America, Europe, all are responsible.”

Susi Air founder and Indonesia’s former maritime affairs and fisheries minister Susi Pudjiastuti told the NZ Herald that Mr Mehrtens was one of the airline’s best pilots.

She said there was yet to be any photographic proof or evidence that Mr Mehrtens was still alive, and there had been no contact with him.

A Papua police spokesman said the area was a highly militarised district with a long history of insurgency in the newly named Highland Papua province.

Police spokesman Ignatius Benny Adi Prabowo said authorities were investigating the incident, and police and military personnel had been sent to the area to locate Mr Mehrtens and his passengers.

“We cannot send many personnel there because Nduga is a difficult area to reach. We can only go there by plane,” he said on Tuesday.

New Zealand’s Ministry of Foreign Affairs and Trade confirmed on Wednesday it was providing consular support to the pilot’s family. It refused to comment further for privacy reasons.

NZ Prime Minister Chris Hipkins said he had been given preliminary details and that the New Zealand embassy in Indonesia was also aware of the incident.

“They haven’t yet fully briefed me on what they know and what they are doing, but I’m aware that they are working on the case,” he told Radio New Zealand.

The remote Nduga Airport, where Mr Mehrtens was taken hostage, and his plane set alight. Photo: Twitter

Indonesia’s easternmost provinces have been wracked with a low-level battle for independence since the resource-rich region was controversially 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.

The increased intensity of these attacks have been enabled by an improved ability to obtain more weapons, including by raiding and stealing from army posts, cross-border purchases and the illegal sale of government-issued weapons, the Institute for Policy Analysis of Conflict said in a report last year.

-with AAP