The risk to countries in the region was now “high”, raised from “moderate”, but the global risk remained “low”.
The reassessment came after the first confirmed case in Mbandaka, a city of around 1.5 million. Previous reports of the disease had all been in remote areas where Ebola might spread more slowly.
“The confirmed case in Mbandaka, a large urban centre located on major national and international river, road and domestic air routes increases the risk of spread within the Democratic Republic of the Congo and to neighbouring countries,” the WHO said.
WHO Deputy Director-General for Emergency Preparedness and Response Peter Salama had told reporters on Thursday that the risk assessment was being reviewed.
“We’re certainly not trying to cause any panic in the national or international community,” he said.
“What we’re saying though is that urban Ebola is very different phenomenon to rural Ebola because we know that people in urban areas can have far more contacts so that means that urban Ebola can result in an exponential increase in cases in a way that rural Ebola struggles to do.”
Later on Friday, the WHO will convene an Emergency Committee of experts to advise on the international response to the outbreak, and decide whether it constitutes a “public health emergency of international concern”.
The nightmare scenario is an outbreak in Kinshasa, a crowded city where millions live in unsanitary slums not connected to a sewer system.
The WHO statement said there had been 21 suspected, 20 probable and 3 confirmed cases of Ebola between April 4 and May 15, a total of 44 cases, including 15 deaths.
Mbandaka had three suspected cases in addition to the confirmed case.
The WHO is sending 7,540 doses of an experimental vaccine to try to stop the outbreak in its tracks, and 4,300 doses have already arrived in Kinshasa. It will be used to protect healthworkers and “rings” of contacts around each case.
The vaccine supplies will be enough to vaccinate 50 rings of 150 people, the WHO said.
As of 15 May, 527 contacts had been identified and were being followed up and monitored, it said.
Reporting by Tom Miles; Editing by Nick Macfie