An Ebola treatment center has now been established and is currently treating 5 patients.
Though global risk remains low, 7 countries have begun screening at airports and ports of entry.
Though the DRC has strong experience with Ebola, the location is very remote and officials don't know the full extent of the outbreak.
A US expert says events are unfolding in ways reminiscent of Guinea in 2014.
A WHO spokesman said ring vaccination would be the recommended approach "if pertinent."
The likely index patient has died, as has the person who drove him to a healthcare facility.
Epidemiologists and responders are en route to the DRC to assist, and Doctors without Borders is sending an emergency team.
Yesterday Brazil's Ministry of Health lifted the state of emergency caused by the Zika virus, due to the country's dwindling case counts of the mosquito-borne disease.
Analysis of 3 urine specimens finds no common pattern.
A WHO official said the hospital cluster involves 17 illnesses, 9 of them fatal, and specimens from 6 people who died tested negative for Ebola.
