A new case was discovered in the health zone known as Iboko - symptoms started June 2 - bringing the overall tally to 62 confirmed or suspected cases and 27 deaths.
It's "extremely likely" that a campaign to vaccinate contacts of people in Mbandaka, a city of more than 1 million, had a positive effect, said Peter Salama, WHO's emergencies chief.
The ongoing Ebola outbreak poses a significant risk of spread to these countries on the basis of proximity to the current outbreak and the current capacity of their health systems to cope with the disease.
World Health Organization is now entering a new phase in which it moves its focus from urban areas to the extremely remote forest areas in north-western Congo where the outbreak began.
"Phase one, to protect urban centers and towns, has gone well and we can be cautiously optimistic".
"There's been very strong progress in the outbreak response, particularly in relation to two of three sites", Salama said.
This is Congo's ninth Ebola outbreak since 1976, when the hemorrhagic fever was first identified.
The outbreak, which began in the remote northwest and was declared in the town of Bikoro, has spread to the city of Mbandaka with a population of more than one million.
An AP report Friday quoted DRC's health ministry saying another Ebola case had been confirmed even as the pace of new cases slows down.
The focus now is on rural isolated communities in the Iboko health zone which will present logistical and other challenges.
On the front lines of the outbreak, health officials use a tool called contact tracing to break the chain of transmission of the Ebola virus.
The disease, which is transmitted by direct contact with the blood and body fluids of infected people or animals, causes severe bleeding and can have a mortality rate of 90%. It's a "major boots on the ground" effort that takes several hours to trace each and every contact, he said.
But the Congolese health ministry said on Wednesday that "no experimental treatment has yet been given to hospitalised patients".
"I am confident that this Ebola outbreak will soon be contained", said Claude Jibidar, WFP's DRC Country Director.
"We're cautiously optimistic but there's a lot of work", said Salama.