At least 12 rangers were killed on Friday in an attack in Virunga National Park, a UNESCO World Heritage site in the restive east of the Democratic Republic of Congo, a park spokesman said.
It was one of the deadliest attacks in the park, Africa's oldest and most biologically diverse protected area.
Another six people were wounded, the spokesman told AFP, adding that four other civilians passing through the area were also targeted.
A security source spoke of a death toll of 13 rangers and five civilians.
Virunga, which is spread over 7,800 square kilometres (3,000 square miles) over the borders of DR Congo, Rwanda and Uganda, is home to a world-famous population of mountain gorillas but has been hit by rising instability and violence.
Inaugurated in 1925, the park has witnessed repeated attacks by rebel groups, militias and poachers.
Visits to the park have been suspended since March 19 for at least 30 days in DR Congo's bid to halt the new coronavirus pandemic.
The park banned visitors between May 2018 to the start of last year after two British tourists were kidnapped there. They were later released but a ranger was killed during the abduction.