Tedros Adhanom Ghebreyesus, the director general of the UN organization, said there were nine cases officially announced by Equatorial Guinea, with seven deaths.
The cases are in three provinces, some 150 kilometres apart, "suggesting wider transmission of the virus," he said.
"WHO is aware of additional cases and we have asked the government to officially report these cases to WHO," he added.
On March 22, the WHO announced that it was aware of 20 cases "probably" caused by this hemorrhagic fever, all deceased.
According to Abdi Mahamud, incident manager, there are "signs of widespread transmission that (worry) us". The appearance of these cases "is more widespread and could manifest itself in several provinces", he added.
"More than the count of the number of cases, it is the geographical extent," according to the WHO official.
Marburg virus is a very dangerous pathogen that causes high fever often accompanied by hemorrhages affecting several organs and reducing the body's ability to function properly.
It is part of the filovirus family, to which the Ebola virus also belongs, which has already caused several deadly epidemics in Africa.
According to Michael Ryan, WHO's director of emergencies, states have clear international obligations in terms of managing health crises.
"When we are in the middle of an outbreak, and we have new and significant information, especially related to laboratory-confirmed cases of dangerous pathogens, ... People need to be aware, alerted and able to take action," he said.
Marburg © virus disease Alain BOMMENEL / AFP/Archives
Any delay in providing such information hampers the response process, he said.
The natural host of Marburg virus is a fruit eating African bat, which carries it but does not get sick.
Animals can transmit it to primates living near them, including humans. Intra-human transmission then occurs through blood contact or with other bodily fluids.
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