Thousands of doses of Ebola vaccine have arrived the Democratic Republic of Congo, as the country battles outbreak of the disease.
Experts warned the virus, which has already killed 23, would be far harder to tackle if it reached the major population centre.
Humans who come into close contact with the blood, organs and bodily fluids of animals infected with the disease are at risk.
"Over the next few days we will be reassessing the projected numbers of cases that we might have and then if we need to bring in more vaccine we will do so in a very short notice".
She says two cases of Ebola are confirmed, 20 are probable and 20 are suspected.
Health workers have recorded confirmed, probable and suspected cases of Ebola in three health zones of Congo's Equateur province, and have identified 432 people who may have had contact with the disease, the WHO said.
The first urban case to be announced threatens to change that.
The vaccine is complicated to use, requiring storage at a temperature between -60 and -80 degrees Celsius. There have also been eight cases and 115 contacts in Iboko health zone.
The World Health Organisation, which is deploying vaccines to the country, previously expressed its concerns of the disease reaching the city of Mbandaka.
Spokeswoman Jessica Ilunga says the vaccines will be sent to Mbandaka and Bikoro, the rural areas where two confirmed cases and several suspected cases have been reported.
The WHO report said 1,500 sets of personal protective equipment and an emergency sanitary kit sufficient for 10,000 people for three months were being put in place.