The worst-ever Ebola outbreak started in December 2013 in southern Guinea before spreading to neighboring West African countries Liberia and Sierra Leone, killing more than 11,300 people out of almost 29,000 registered cases.
The WHO said the risk of the disease spreading was "high" and announced it was preparing for the "worst case scenario".
"Our current estimate is we need to vaccinate around 8,000 people, so we are sending 8,000 doses in two lots", he told Reuters in Geneva.
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.
"In our experience, for each confirmed case of Ebola there are about 100-150 contacts and contacts of contacts eligible for vaccination", Jasarevic said.
The first urban case to be announced threatens to change that. The vaccine will be reserved for people suspected of coming into contact with the disease, as well as health workers. The Congo Republic is on the other side of the river. There have also been eight cases and 115 contacts in Iboko health zone. The World Health Organization, which on Wednesday deployed the first experimental vaccines in the vast central African country, had expressed concern about the disease reaching Mbandaka, which would make the outbreak far harder to tackle.
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.