The diagnosis was made from blood samples taken from the victims - mostly gold miners - and tested by the National Institute of Virology in South Africa.

East Africa Correspondent Martin Dawes: "Extremely high fatality rate"

Congolese state television said on Wednesday the death toll from the fever, which broke out around the town of Durba in rebel-held territory in January, had risen to 72.

However the WHO says the "latest figures are an estimated 76 cases with 52 deaths".

WHO spokesman Gregory Hartl said some of the gold miners had died of other causes. "Some deaths have now been excluded as having been caused by Marburg. They may have been malaria or something else," he said.

Victims bleed to death

Marburg victims have a high fever and bleed to death from body orifices. The symptoms led to speculation that this was an outbreak of the much-feared Ebola virus, but the WHO ruled that out earlier in the week.

As with Ebola, there is no specific treatment or vaccine for the Marburg virus.

Congolese Health Minister Mashaka Mamba said it was possible the epidemic was tailing off, as there were only five registered active cases of the disease in the health centres in Durba and Wtasa, the two most affected towns in north-east Congo.

Most of the victims were gold miners, leading to speculation that the men may have come in contact with the Marburg virus in the course of their work.

The WHO says it will be trying to find the exact source of the outbreak, but BBC East Africa correspondent Martin Dawes says access will be difficult as the area is remote and at war.

History of the virus

The virus was first identified in 1967, in the North German town of Marburg, after a laboratory worker who was taking blood from African monkeys became ill and died.

Even in the best hospitals, with the most modern facilities, the virus can be expected to kill a quarter of those who go down with it.

It is extremely rare and, for that reason alone, researchers will want to find out as much as possible about this latest outbreak.