The number of known Ebola cases has now increased to 17, with the risk of the deadly virus spreading further dubbed "high." The WHO has not yet declared an international health emergency.
Congo's health ministry issued a statement late Friday confirming three new cases of lethal Ebola virus in Mbandaka city. A single case of Ebola in Mbandaka, a city of 1.2 million people, was confirmed last week.
There are now 17 confirmed and 43 possible Ebola cases in the Democratic Republic of the Congo. At least one person has died of the disease.
Health officials are trying to locate more than 500 people who they believe have been in contact with those feared infected. But the task is extremely challenging in Mbandaka, a densely populated city that is an hour's flight from the capital Kinshasa.
As of May 15, health officials had identified 527 contacts between people believed to be infected with Ebola. Earlier this week, more than 4,000 doses of an experimental Ebola vaccine arrived in Congo as part of the Unite Nation's efforts to stem the outbreak.
Ebola is a near-fatal disease that causes a hemorrhagic fever that often leads to massive internal bleeding.
This is the ninth Ebola outbreak in Congo in four decades. The last major Ebola outbreak hit the eastern African countries of Guinea, Liberia and Sierra Leone, killing some 11,000 people.
Despite the rapid spread of the disease, the World Health Organization (WHO) on Friday decided against declaring an international health emergency.
There is "strong reason to believe this situation can be brought under control," said Dr. Robert Steffen, who chaired the WHO expert meeting on Friday. But without a vigorous response "the situation is likely to deteriorate significantly," he added.
The WHO acknowledged that "the confirmed case in Mbandaka, a large urban center 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 neighboring countries."
"We're certainly not trying to cause any panic in the national or international community," Peter Salama, the WHO's deputy director general for emergency preparedness and response, told reporters Thursday.
"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."
"This is a concerning development, but we now have better tools than ever before to combat Ebola," said Tedros Adhanom Ghebreyesus, the WHO's director-general. "WHO and our partners are taking decisive action to stop further spread of the virus."