The illness causes internal bleeding and is easily transmitted through contact with small amounts of bodily fluid. Twenty-three deaths have been reported in Bikoro in connection with the latest outbreak. This is the ninth Ebola outbreak in Congo since the 1970s, and the first since May of past year, when five confirmed cases resulted in four deaths.
One case has been confirmed in Mbandaka a city of around 1 million is top priority as it is connected to the capital of Kinhasa, a city of 10 million increasing the transmission risk significantly and giving rise to the fear that the outbreak to turn into an epidemic. Its flu-like symptoms make it hard to diagnose.
World Health Organization has also confirmed that the current outbreak is the same strain of the virus that broke out in West Africa in 2013 and went on to kill more than 11,300 people, the deadliest ever Ebola epidemic.
The WHO is also deploying 30 experts to Mbandaka to "conduct surveillance in the city and is working with the Ministry of Health and partners to engage with communities on prevention and treatment and the reporting of new cases".
The biggest recent trials with the vaccine took place in a coastal region of Guinea at the tail end of a major outbreak in 2015. "Now that we are facing the Ebola virus we must use all the resources we have". But "urban Ebola can result in an exponential increase in cases in a way that rural Ebola struggles to do".
"This is a concerning development", World Health Organization director-general Tedros Adhanom Ghebreyesus said in the statement.
An outbreak between 2014 and 2016 in west Africa was the worst ever, and killed more than 11,000.
Forty-four cases have been reported in the outbreak so far, including three confirmed, 20 probable and 21 suspected, according to the WHO's tally.
They were recorded in Congo's Equateur province. Since then, an experimental vaccine has been developed, and 4,000 doses have arrived in the Congo, with thousands more to follow.
"Now we have confirmation that we have urban ebola, which is a very different animal to rural ebola".
The vaccine has been shown to be highly effective against Ebola. The transportation of the vaccine is slowed by the need for it to be stored at extreme, subzero temperatures.
Professor William Schaffner said a swift response was needed to halt the virus from claiming more lives. Basic infrastructure - hospitals, roads, electricity - is a major obstacle in remote areas.
The worldwide response to the current Congo outbreak has been substantial and is expected to grow in size and urgency after the announcement of a confirmed urban case. Health care workers have been among those infected, according to the country's health ministry. These can include chimpanzees, gorillas, monkeys, antelope and porcupines. As long as humans come in contact with them, there is always a possibility that Ebola could return.