"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 most severe strains of Ebola have had a fatality rate of up to 90 percent and there is no vaccine, cure or specific treatment.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.
"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 on Tuesday, 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.
Twenty of the cases have been in the capital Conakry, a sprawling port city on Guinea's Atlantic coast that is home to between 1.5 million and two million people.
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."- 'Need to remain vigilant' -
In Liberia, there have been 21 cases, including 10 fatalities, of which five have been confirmed as Ebola.
There have also been two suspected cases in Sierra Leone, affecting people who are believed to have caught the disease in southern Guinea but who died over the border.
In Mali, there have been nine suspected cases, with tests so far showing two of them did not have the fever.
A suspected case in Ghana meanwhile turned out not to be Ebola.
"What's most important is the trend and the geographical spread of the infection," cautioned Stephane Hugonnet, a WHO medical officer who returned from southern Guinea this weekend.
"Obviously there is a risk that other countries might be affected, so we absolutely need to remain vigilant," he told reporters.
Ebola was first recorded in the 1970s in what is now the Democratic Republic of Congo.
The largest-ever outbreak on record was in 2000-2001 in Uganda, with 425 cases, half of whom died, according to WHO data.
Ebola leads to haemorrhagic fever, which causes muscle pain, weakness, vomiting, diarrhoea and, in severe cases, organ failure and unstoppable bleeding.
Patients' chances of survival increase if they are kept hydrated and receive treatment for secondary infections, according to experts.
The virus can be transmitted to humans from wild animals, and between humans through direct contact with another's blood, faeces or sweat.
Sexual contact, or the unprotected handling of contaminated corpses, can also lead to infection.