Militia groups in eastern Democratic Republic of Congo have killed two park rangers in one of the world’s last sanctuaries for the endangered mountain gorilla, the park said on Monday. The third ranger is missing after the attacks on Saturday in Virunga National Park, Africa’s oldest national park and a UNESCO World Heritage site.

Virunga is home to roughly half the world’s remaining 900 mountain gorillas and was the subject of an Oscar-nominated documentary film in 2014. The park said in a statement four ranger posts on the shores of Lake Edward were attacked by more than 120 rebels belonging to what appears to be a new coalition of militia groups.  Dozens of armed groups operate in and around the 7,800-square-kilometre park, which abuts Congo’s borders with Rwanda and Uganda.

Two rangers were captured by the rebels and killed and a search is ongoing for the missing ranger, the statement said. It said rangers and Congolese soldiers had reclaimed the overrun positions, killing one rebel and capturing another. These rangers were killed in situations that may amount to war crimes in any other conflict,” park director Emmanuel de Merode said in the statement. More than 150 Virunga rangers have been killed over the last decade, de Merode added, many battling poachers who target the park’s gorillas, elephants and other rare animals. The rangers’ presence in the park is resented by many of the more than four million people who live near it but are prevented from cultivating its rich soil. The Rwandan genocide next door and collapse of Congo’s government in the mid-1990s precipitated years of war in the east that killed millions, mostly from hunger and disease. Armed groups continue to terrorize local populations as they compete over gold, timber and other resources.

International donors are investing over $150 million in and around the park to build hydro plants and tourist infrastructure that they hope can bring sustainable development to the area.But security has deteriorated in recent months with a spate of kidnappings and inter-ethnic fighting that has led humanitarian organizations to limit their movements.