Keiji Fukuda, assistant director-general of the World Health Organisation (WHO), said the agency was concerned about the spread of the outbreak from its hub in the forests of southern Guinea.
"We have not had an Ebola outbreak in this part of Africa before," said Fukuda, whose agency has rushed scores of aid workers to the region to contain the epidemic.
"This is one of the most challenging Ebola outbreaks we have ever faced," he said.
The outbreak has sparked fear in Guinea, where a mob in the south of the country last week attacked international aid workers, who they blame for bringing the haemorrhagic fever.
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"These kind of outbreaks are often surrounded by a great deal of fear and anxiety," Fukuda said.
According to fresh figures released by the WHO today, there have been 157 suspected cases in Guinea -- the ground zero of the disease -- 101 of them fatal. Of those, 67 have been confirmed as Ebola victims by laboratory tests.
Other countries across west Africa have been bracing against the epidemic, with Senegal closing its border with Guinea to stop it spreading.
"Ebola is clearly a severe disease. It's an infection with a high fatality rate. But it's also an infection that can be controlled," Fukuda said.
"Our main purpose is really to support the affected countries, in terms of trying to prevent infections, stop infections, stop the outbreak, and then make sure that those who are sick get the best possible care.