Two park rangers and five militiamen were killed in clashes in Africa's oldest wildlife reserve, the Democratic Republic of Congo's Virunga National Park famed for its mountain gorillas, the army said Monday.

"Two park rangers were killed" on Saturday in a joint attack by the Mai-Mai militia and fighters from the Democratic Forces for the Liberation of Rwanda (FDLR), an ethnic Hutu militia based in eastern DRC, Capt. Guillaume Djike, an army spokesman in the region, told AFP.

The army launched a counterattack and "killed five assailants", he said, adding that the militias wanted to set up in the area to carry out "illegal fishing in Lake Edward."

"The search operation is continuing in the region", infested with rebels and armed militias, about 100 kilometres (60 miles) north of Goma, the capital of the turbulent province of Nord-Kivu.

A statement from park authorities said the two rangers were captured by the rebels "before being summarily executed". A third ranger is missing, it said.

"Four ranger positions on the shores of Lake Edward in the Central Sector were attacked by Mai-Mai militia over the weekend", the statement said.

"The coordinated attacks were carried out by over 120 rebels, beginning in the early hours of Saturday morning. It is believed this number represents a new coalition of Mai Mai rebels specifically targeting Virunga's rangers."

A total of 150 rangers have now been killed in the past decade in Virunga, the park's director Emmanuel de Merode was quoted as saying in the statement.

"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", he said.

Opened in 1925, during Belgian colonial rule, Virunga National Park is home to several threatened species, including its emblematic mountain gorillas.

The 7,800-square-kilometre (3,000 square mile) UNESCO world heritage site, which borders Rwanda and Uganda, reopened to tourists last year after being closed for two years because of militia violence in the region.

The two eastern Kivu provinces, North and South, have been chronically unstable since two wars wracked the vast country between 1996 and 2003, drawing in armies from neighbouring and southern African countries, who fought in part over access to vast mineral wealth.