Three patients escaped from quarantine in the city of Mbandaka in north western Democratic Republic of Congo.
The death toll of the outbreak has reached 27 as health chiefs crackdown on the virus.
World Health Organisation bosses have previously warned spread in the city could mean a repeat of the 2014 outbreak – which killed over 11,000.
A spokesman for WHO said:
“The appearance of a confirmed case in Mbandaka is very concerning.
“WHO and partners are working together to rapidly scale up the search for all contacts of the confirmed case in the area.
“Controlling a virus once it has reached a large urban area is particularly challenging.
“The last big Ebola outbreak in 2014-15 showed the devastation that can occur when the virus gains a foothold in an urban setting.”
Reuters reports that two of the three have been found dead, while a third patient has been found alive.
Medecins Sans Frontieres’ (MSF) mission in the city confirmed the incident as they work to treat cases of Ebola.
WHO have shipped more than 8,600 vaccines to DRC, in a desperate attempt to prevent the spread of Ebola.
Another 8,000 are expected to arrive in coming days.
Doctors have hailed the latest dose of the vaccine a “paradigm shift” in fighting the disease.
And medical chiefs at WHO have claimed the world is much more prepared than in 2014.
Aid agencies are battling to persuade sceptical residents about the severity of an outbreak that has killed 27 since April.
Some people are refusing medical care and turning to preachers and prayers to chase away the threat.
Pastors at churches have been encouraging people “pray away” the Ebola instead of receiving vaccines.
Ebola is also believed to be a “curse on those who ate stolen meat”.
African health officials have said they were preparing to send anthropologists to the DRC to help with an Ebola vaccination campaign.
Prototype vaccine will first be given to frontline health workers and then to people who have been in contact with Ebola cases.
John Nkengasong, head of Africa Centres for Disease Control said: “If we do not handle communication well, the vaccination programme may suffer.
“So we are also assessing how in the next two weeks or so to deploy anthropologists to support the vaccine efforts.”
Mr Americana, Overpasses News Desk
May 23rd, 2018