Keiji Fukuda, assistant director-general of the World Health Organization (WHO), said the agency was concerned about the spread of the virus from its epicentre 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 most severe strains have had a 90 percent fatality rate, 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.
"It's absolutely critical to get out as much accurate information as possible to communities and the countries affected, to reduce the rumours, so that people have facts to work with," Fukuda said.
"Ebola is clearly a severe disease," he said.
" It's an infection with a high fatality rate, but it's also an infection that can be controlled."
According to fresh WHO figures released on Tuesday, there have been 157 suspected cases in Guinea, 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 and home to between 1.5 million and two million people.
The WHO has not recommended any trade and travel restrictions for Guinea.
But other countries across west Africa have been bracing against the epidemic, with Senegal closing its border with Guinea.
"We have everything in place to take measures against Ebola," Senegal's Health Minister Eva Marie Coll Seck said on Tuesday after visiting the port and airport in the capital, Dakar..
"We have a well-oiled system, which we are perfecting daily."
The disease is a particular concern for Senegal because it is a leading tourist destination in the region, with arrivals topping one million in 2011, according to the World Bank.
In Liberia, there have been 21 cases, including 10 fatalities, of which five have been confirmed as Ebola.