There are now 14 confirmed Ebola cases in Congo's latest outbreak as health officials rush to contain the often deadly virus in a city of more than 1 million.
The Ebola virus disease, which most commonly affects people and nonhuman primates such as monkeys, gorillas and chimpanzees, is caused by one of five Ebola viruses.
A panel will decide on Friday whether to declare a "public health emergency of global concern", which would trigger a larger response.
The reassessment came after the first confirmed case in Mbandaka, a city of around 1.5 million in the northwest.
The city of about one million people is a transport hub on the River Congo, prompting fears that the virus could now spread further, threatening the capital Kinshasa and surrounding countries.
"The arrival of Ebola in an urban area is very concerning and WHO and partners are working together to rapidly scale up the search for all contacts of the confirmed case in the Mbandaka area", said Dr Matshidiso Moeti, WHO regional director for Africa.
She said her ministry would continue to monitor the situation at all levels and will update the public as the epidemic unfolds.
"There is absolutely no reason why this current outbreak should grow any further", said Dr Kamalini Lokuge, the head of the Australian National University's Humanitarian Research Program. Vast, impoverished Democratic Republic of Congo has contained several past Ebola outbreaks but the spread of the hemorrhagic fever to an urban area poses a major challenge.
An outbreak in West Africa that began in 2014 left more than 11,000 dead across six countries, and was not declared officially over by World Health Organization until the beginning of 2016.
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.
"This is a concerning development, but we now have better tools than ever before to combat Ebola", WHO director-general Tedros Adhanom Ghebreyesus said.
He said the disease could have been taken there by people who attended the funeral of an Ebola victim in Bikoro, south of Mbandaka, before travelling to the city. Ebola is a highly infectious disease spread through contact with even small amounts of bodily fluid of an infected person.
It is the ninth time Ebola outbreak that has been recorded in the DRC, whose eastern Ebola river gave the deadly virus its name when it was discovered there in the 1970s, and the outbreak comes less than a year after one that killed eight people.