Last month's mysterious attack, in which the carcasses were left inside Virunga National Park rather than eaten or taken to be sold as bush meat, dealt a blow to efforts to save the species from extinction.

Fewer than 700 mountain gorillas remain, all in central Africa near the intersection of Congo, Uganda, Rwanda and Burundi - an area rocked by years of wars and where armed militias still roam the forests after Congo's 1998-2003 war, which brought six foreign armies into the country.

A team of UNESCO investigators were at the Bukima ranger station, 30 km (20 miles) north of the provincial capital Goma, on Thursday when a local ranger found the remains.

"The baby that was with her was not old enough to live without its mother," Yvette Kaboza, a programme specialist with UNESCO's world heritage centre, told Reuters.

Conservationists rescued another orphaned baby, a female, just days after the attack, and took her away to a sanctuary in Goma because she was not yet weaned and too young to survive in the wild without her mother.

"Effectively, this means that not only are there six that are now dead, but there will now be a group of 12 gorillas that may not carry on into the next generation, said Gerard Collin, a consultant with the UNESCO team.

So far this year, nine mountain gorillas have been killed in North Kivu.

Two adult males, known as silverbacks because of their grey colouring, were killed and eaten by rebels living off the land.

A third, a female, was shot in the back of the head in what conservationists said was an "execution-style" killing. Rangers found her baby clinging to her body, suggesting she was not killed for bush meat or the lucrative trade in primate infants.

Some conservationists say they suspect the killings are linked to a power struggle between local government agents trying to save Virunga, Africa's oldest national park, and those engaged in the illicit trade in charcoal made from its trees.

Under Congo's late dictator Mobutu Sese Seko, Virunga was a major tourist draw, but years of insecurity and the 1998-2003 war that killed an estimated 4 million people mainly through hunger and disease, have led to a dwindling number of visitors.