The World Health Organisation (WHO) said on Friday that at least 1,500 people had been potentially exposed to the deadly Ebola virus in the Democratic Republic of Congo's North Kivu region, where insecurity prevents aid workers from reaching some areas.
But it expected more people to become infected and could not be sure that it had identified all chains by which the virus is spreading in the eastern part of the country beset by militia violence.
More than 500 people including health workers have been vaccinated against the disease in Congo's latest outbreak, marked by a total of 78 confirmed and probable cases, including 44 deaths, the WHO said.
Some 1,500 people have been identified as contacts of people infected with the disease that causes fever, vomiting and diarrhoea.
"We don't know if we are having all transmission chains identified. We expect to see more cases as a result of earlier infections and these infections developing into illness," WHO spokesman Tarik Jasarevic told a Geneva news briefing.
"We still don't have a full epidemiological picture... The worst case scenario is that we have these security blindspots where the epidemic could take hold that we don't know about," he said.
The outbreak has spread from its epicentre in North Kivu province to neighbouring Ituri province after an infected person returned home, Congo's health ministry said this week, complicating containment in a region beset by widespread militia violence.
The WHO said it had deployed more than 100 experts including epidemiologists to the towns of Beni and Mangina, to oversee tracing, vaccination and safe burials.
But in line with UN security, there are so-called 'red zones' near the epicentre of Mangina which aid workers cannot enter, Jasarevic said.
"When decisions are being made, the priority is the security of all responding teams," he added.