KINSHASA (Reuters) - Congolese rebels have shot and butchered a rare mountain gorilla, raising fears for a tiny population that has clung on through years of warfare in central Africa, conservationists said on Wednesday.

Just 700 mountain gorillas survive, more than half of them in Virunga National Park in eastern Democratic Republic of Congo. The east of the country bore the brunt of a 1998-2003 war and humanitarian disaster that has killed some 4 million people.

"In a population this small, every individual counts -- and the loss of a trusting young silverback is tragic on many levels," Ian Redmond, chief consultant for the United Nations Great Apes Survival Project (GRASP), said in a statement.

Adult male gorillas are known as silverbacks because of their grey colouring.

The statement was issued by Nairobi-based conservation group Wildlife Direct which supports gorilla protection efforts in Virunga, Africa's oldest national park and a United Nations World Heritage Site.

Wildlife Direct accused fighters loyal to renegade Congolese general Laurent Nkunda of shooting the silverback last week, and said they ordered a local farmer to help butcher it. Primates and other mammals are prized in parts of Africa as "bush meat".

"The future survival of this species is now under threat, and I fear that this recent attack on the gorillas could signal a wave of such killings if immediate action is not taken to remove Nkunda's and his troops from their habitat," Robert Muir, of the Frankfurt Zoological Society, said in the statement.

Wildlife Direct said the gorilla had been one of a group which were used to humans because of regular trips by tourists before the war broke out. Congo held landmark elections last year, but militia violence continues in eastern areas.

Conservation efforts have helped the mountain gorilla population grow by 14 percent since the war began.

Wildlife Direct said the gorilla was killed just 600 metres (yards) from one of several patrol posts which rangers abandoned in November due to attacks and looting by Nkunda's fighters.

Some 97 rangers have been killed since 1997 protecting Virunga from poachers, it said. The park spans Congo, Rwanda and Uganda and is home to 380 mountain gorillas.

The other population, of 320, is in the nearby Bwindi National Park in Uganda.