These would be given as a priority to people in Mbandaka who had been in contact with those suspected of carrying the Ebola virus before people in any other affected area, Mr Salama said.
In this photo taken Saturday, May 12, health workers don protective clothing as they prepare to attend to suspected Ebola patients at Bikoro Hospital in Bikoro, Congo.
"This outbreak is very close to the Republic of Congo and Central African Republic".
The greatest cause for concern is a confirmed case of the deadly disease in Mbandaka, a city of about a million people and a transport hub on the Congo River in the north-west of the country.
The global response to the current Congo outbreak has been substantial, and is expected to grow in size and urgency after the announcement of a confirmed urban case. "Now that we are facing the Ebola virus we must use all the resources we have".
The deployment of this experimental vaccine will prove a major test for not only the efficacy of the vaccine but for global health strategies created to respond to major viral outbreaks such as this.
The UN agency is also partnering with nongovernmental organizations, including Médecins Sans Frontières, to ensure that health facilities are ready to treat patients in isolation wards.
Since the outbreak began, there has been a total of 45 cases: 14 confirmed, 21 probable and 10 suspected.
Traditionally, health officials have largely relied on something called contact tracing to contain Ebola outbreaks.
This new case is linked with the epicentre of the epidemic, the east of Lake Tumba.
Ebola cases on rise, reach 14 in DRC
An infected person is estimated to have between 100-150 "contacts" with other people per day, a figure that may be higher in densely populated areas, Jasarevic added.
Peter Salama, in charge of emergency response at the World Health Organization, said the spread to a city complicated the fight against Ebola, which depends on identifying and isolating suspected cases.
Cases emerged in a rural area with one now confirmed in the city of Mbandaka.
"We are entering a new phase of the Ebola outbreak that is now affecting three health zones, including an urban health zone", the Democratic Republic of Congo's Minister of Health Oly Ilunga Kalenga said in a statement in French late Wednesday.
The aid group Medecins Sans Frontieres said 514 people who are thought to have been in contact with infected people were now being monitored.
The first human outbreaks of Ebola occurred in 1976, one in the north of what is now Congo and in the region that is now South Sudan. The virus can be fatal in up to 90 percent of cases, depending on the strain.
The 2014-16 West Africa outbreak became particularly deadly when it spread to the capitals of Guinea, Sierra Leone and Liberia.
Symptoms include fever, vomiting, diarrhea, muscle pain and internal and external bleeding.
Eleven people were treated for Ebola in the United States in 2014, two of whom died, according to the Center for Disease Control and Prevention.