There are only about 700 mountain gorillas left in the world, conservation group World Wildlife Fund (WWF) International said on Monday.
Any further loss of remaining habitat in the lush mountains and volcanoes straddling Rwanda, Uganda and the Democratic Republic of the Congo can push them closer to extinction.
"Over the last two months, 1500 hectares (15 sq km) of prime mountain gorilla habitat have been cleared by illegal settlers in Virunga National Park, a World Heritage Site," Swiss-based WWF said in a statement.
"Since April, convoys of people from Rwanda and the Democratic Republic of Congo have destroyed large tracts of the park, home to the mountain gorilla and other endangered species, to create agricultural and pastoral land," it said.
In all of Virunga there is only about 425 sq km of suitable gorilla habitat and so the loss of even 15 sq km is huge.
"This is particularly significant since it is encroaching on an area where gorillas live. There are three family groups in that area comprising about 50 animals," said Dr Peter Stephenson, WWF's African Great Apes Programme coordinator.
WWF said evidence of the deforestation was uncovered by the Congolese Institute for the Conservation of Nature and that most of the damage took place from early May to June.
"Several thousand people moved in to the area to farm illegally in Virunga. The forest has been entirely cut down and turned into timber or charcoal," WWF said.
"Several thousand people moved in to
the area to farm illegally in Virunga. The forest
has been entirely cut down and turned into timber or charcoal"
Dr Peter Stephenson,
WWF programme co-ordinator
WWF said recent meetings between Congolese and Rwandan officials had led to a cessation in deforestation.
However, the wildlife group says it remains concerned that it could start again.
"If the deforestation continues then some of the animals will be cut off from the rest of the park," Stephenson said.
The border area has been a scene of tension in recent weeks after Congo accused Rwanda, which has twice invaded its giant neighbour in the past eight years, of backing renegade troops who briefly seized an eastern Congolese town in early June.
Rebel groups have used the gorillas' forest home for bloody incursions into all three countries and military activity is another threat to their survival.