The execution of two rangers in Virunga National Park by anti-government rebels has pushed losses of the guardians of this vast reserve in eastern Democratic Republic of the Congo above 150 during the past decade. A third ranger went missing in the attack.

The World Heritage Site is home to an impressive array of landscapes and wildlife, including famed mountain gorillas, but it’s been under a pall of violence because of instability in the region.

This latest attack represents an emboldened threat to the park, officials warn. During the past few years, the rebels have repeatedly made violent attempts to control Lake Edward in the park, which is used as a transportation hub. Fighting there in July 2015 among the rebels, rangers, and the Congolese Army resulted in the death of a ranger and injuries to several others .

"Our rangers stand fearlessly on the frontline of the protection of civilians and their resources knowing that they are at risk of extreme violence," park director Emmanuel de Merode said in a statement following the violence, which took place in the park's central section from March 12 to 13.

Rangers Mulonga Mulegalega Fidele and Mumbere Muvesevese Vevant were reported killed. (The park is asking for donations to combat the threat now.)

"The situation is deplorable, and we must honor their sacrifice by attempting to reduce this threat of violence," said de Merode, who has overseen the strengthening of the ranger force and a corresponding rebound of wildlife over the past few years.

"These rangers were killed in situations that may amount to war crimes in any other conflict," he said.

Four ranger positions on the shores of Lake Edward in the park were attacked by several heavily armed groups, which park officials have identified as a coalition of some 120 Mai-Mai rebels. "Both rangers were captured by the rebels before being summarily executed," the park says in a statement.

The rangers and the Congolese National Army launched a counterstrike, retaking their positions and retrieving the bodies of their fallen comrades. One rebel was killed, and another was arrested. The search for the missing ranger continues.

Watch: Virunga's warden on the dangers of protecting the park.

"Despite the wide progress we make here in many areas, we cannot sustain these kind of losses in what is still the most dangerous conservation job in the world," de Merode said.

Last year, de Merode and his colleague Innocent Mburanumwe were named the Rolex National Geographic Explorers of the Year on behalf of all the rangers of Virunga National Park. The 3,000-square-mile (7,800 square kilometers) reserve was established in 1925 as Africa's first national park. Violence has occasionally broken out there over control of its resources and at the instigation of opposing armed groups in the area.

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