Five wildlife rangers and a driver guarding one of the world's most important refuges for mountain gorillas and other critically endangered species have been killed in an ambush.

Authorities in the Virunga National Park, the Democratic Republic of Congo's famed haven for gorillas, said the men were gunned down by militia men early on Monday near the border with Uganda.

"Virunga National Park is deeply saddened to confirm reports of an attack on our staff today," the park said in a statement.