Soon after dawn on a Wednesday morning in mid-June wildlife rangers on patrol in Garamba National Park, a vast swathe of northeastern Democratic Republic of Congo, were ambushed.

A group of armed men attacked the anti-poaching patrol killing one ranger and two Congolese soldiers, each shot dead at close-range.

"They knew what they were doing," said Garamba's chief warden.

"There were 15 of them, judging from the tracks, who clearly had a military background, they were not simple poachers. They killed our guys from five metres away, execution style."

According to the chief warden the killers were members of South Sudan's national army (SPLA), who now pose one of the biggest threats to DR Congo's dwindling population of elephants, and the rangers protecting them.

"The SPLA target the rangers specifically, they set up ambushes to hunt us down. It is unlike anything I have ever seen," he said.

The chief warden's account is contained in a new report by Ledio Cakaj, a researcher for The Enough Project pressure group, investigating the role of armed groups in the illegal ivory trade.

The report titled "Tusk Wars" finds evidence that Joseph Kony's rebel Lord's Resistance Army (LRA) is trading ivory for weapons with Sudanese soldiers and merchants. While Khartoum-backed Janjaweed militias are also involved in poaching, it blames armed forces from South Sudan for the bulk of the killing.

"According to park rangers, the biggest current threat to both elephants and wildlife rangers in Garamba are poachers from South Sudan, reportedly part of the police or the national army," Cakaj said.

- 'Ready to die for ivory' -

Roughly twice the size of Luxembourg, Garamba loses up to 150 elephants each year with park authorities estimating South Sudanese poachers are responsible for 80 percent of the killing.

"The South Sudanese are ready to die for ivory, rather than spare the elephant or the ranger. Because it is so lucrative," said Garamba's chief warden.

There are believed to be thriving illegal ivory markets in the South Sudanese towns of Yei and Maridi where a kilo (2.2 pounds) of ivory can fetch hundreds of dollars.

In South Sudan, too, elephant populations have been hit by a rise in poaching by armed groups since civil war began in December 2013, according to surveys conducted by the Wildlife Conservation Society.

Cakaj said that some poachers in Garamba were spotted wearing SPLA uniforms, while SPLA uniforms were also found alongside elephant tusks in poaching camps, but he said there remains "a lack of clarity" as to whether the soldier-poachers are serving members, renegades or deserters.

South Sudanese soldiers are also part of a 2,500-strong US-trained African Union Regional Task Force meant to hunt down the LRA.

The Enough Project estimates Kony to have just 120 fighters under his command, while President Obama has deployed roughly the same number of US Special Forces advisors to stop him.

Despite its diminished size, the group continues to pose a threat, "with 150 recorded attacks and 500 abductions of civilians for the first eight months of 2015 and 200,000 people displaced," according to the report.

Kony himself is believed to be hiding in a Sudan-controlled enclave called Kafia Kingi where he stockpiles his ivory before trading it for food, clothes and ammunition.