The WHO has sent 150 experts to the region to help prevent the spread of the virus but admits " there has been significant increase in the number of daily reported cases and deaths".
Dr Luis Sambo, the WHO's regional director for Africa, said: "This is no longer a country-specific outbreak, but a sub-regional crisis that requires firm action.
"WHO is gravely concerned of the on-going cross-border transmission into neighbouring countries as well as the potential for further international spread.
"There is an urgent need to intensify response efforts...this is the only way that the outbreak will be effectively addressed."
The charity Medecins Sans Frontieres (MSF) has already warned that the Ebola outbreak is out of control.
It says the epidemic will spread further unless there is a stronger international response.