West Africa's Ebola outbreak is among the "most challenging" ever to strike since the disease emerged four decades ago, the WHO said, as the suspected death toll from the virus hit 111.
"This is one of the most challenging Ebola outbreaks we have ever faced," said Keiji Fukuda, assistant director-general of the World Health Organisation.
WHO said on Tuesday that it expected to have to confront an outbreak of the deadly virus Ebola in West Africa for the next two to four months, the Reuters news agency reported.
But the United Nations agency said it was not recommending any travel restrictions to Guinea, which has a total of 157 suspected and confirmed cases including 101 deaths, or Liberia which has 21 suspected and confirmed cases including 10 deaths.
"We fully expect to be engaged in this outbreak the next two to three to four months before we are comfortable that we are through it," Fukuda told a Geneva news briefing.
WHO medical officer Stephane Hugonnet, just back from Guinea, said: "In Guinea Forestiere, the outbreak is not over, this is the epicentre. As long as it is not over there, there will be cases exported to the rest of the country."
Some 50 foreign experts have been deployed to help with infection control measures at hospitals and to trace 600 known contacts of infected patients, he said.
Most new cases were linked to known transmission chains, which he said was "reassuring".