The health agency has raised its earlier assessment after one patient in a major city was confirmed as having the disease.
Although the global risk of an Ebola outbreak remains low, the WHO says the risk to countries neighbouring the Congo is now “moderate”.
At an emergency meeting on Friday, WHO stopped short of calling the outbreak a “public health emergency of international concern”.
There are 45 cases with 14 confirmed and 25 dead, according to WHO.
On Thursday, a case was confirmed in Mbandaka – a city of about 1.5 million people.
The city lies about 93 miles from Bikoro, a remote area where the current outbreak was announced last week.
Now the disease has been reported in a highly populated urban area, it is expected to spread more quickly.
The DRC implemented an “action plan” boosting safeguards in the northwestern city.
The UK’s Department for International Development has announced that three UK experts funded by UK aid are deploying to the DRC to help tackle the Ebola outbreak.
“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 has stressed that the agency is “certainly not trying to cause panic in the national or international community”.
He added: “What we are 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.”
If the outbreak reaches Kinshasa, a crowded city where millions live in unsanitary slums not connected to a sewerage system, the situation will become much more concerning.
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 health workers and “rings” of contacts around each case.