That is despite the threat posed by poachers and armed groups in the Virunga Massif,a spine of volcanic mountains in the western Rift Valley straddling eastern Democratic Republic of Congo, Uganda, and Rwanda, where most of them reside.

The latest census put them at 1,004 individuals: 604 in Virunga and 400 in Uganda's nearby Bwindi Impenetrable National Park, Joel Wenga Malembe, spokesman for the Congolese Institute for the Conservation of Nature, told Reuters.

The last survey, in 2010, found just 786 of this critically endangered eastern gorilla sub-species, of which 480 were in Virunga.

"These numbers are truly remarkable, far exceeding our expectations, and are the result of a collaborative, three-country effort with governments and partners all playing an important role," Mike Cranfield, of charity Gorilla Doctors, said in a statement.

Partly because they are so rare, the mountain gorillas are a major source of tourist revenue for the three nations, and their habitat supports other rare species found nowhere else such as golden monkeys.

But they are under constant threat of encroachment by farmers in one of Africa's most densely populated rural regions and hunting for bushmeat or bizarre trophies sold abroad.

Oil companies a hiccup

Despite countries in the Great lakes region wanting to earn foreign exchange from tourism, oil companies have shown increasing interest in prospecting for oil, thus hindering conservation efforts.

For instance, the Democratic Republic of Congo (DRC) invited oil companies to start drilling in Virunga, Africa's largest tropical rainforest reserve, which conservationists worry will put the environment in greater danger.

Adding to Virunga's woes, rangers banned tourists this month from entering the park during investigations into the kidnapping of two Britons by an armed militia. Mountain gorillas are popular with tourists. The region's population, however, dropped sharply in the past century because of poaching, illness and human encroachment.