Militia violence in the Democratic Republic of Congo (DRC) is preventing aid workers from accessing potential Ebola victims, according to the World Health Organization (WHO).
A total of 78 confirmed and probable cases of the disease have been recorded in northeast DRC since August 1, WHO said on Friday.
The latest outbreak has claimed so far 44 lives (17 confirmed and 27 probable), it added.
At least 1,500 people have been potentially exposed to the deadly virus in North Kivu province but violence in the region means officials cannot be sure if they have identified all the chains by which it is spreading in the east of the vast country.
In the area, health workers are being forced to navigate their response among more than 100 armed groups.
Tedros Adhanom Ghebreyesus, the WHO chief, said the region was sprinkled with so-called "red zones", or inaccessible areas.
"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 news briefing in Geneva.
"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 is spreading across the lush farmlands of eastern Congo and has already reached neighbouring Ituri province.
Ebola causes serious illness including vomiting, diarrhoea and in some cases internal and external bleeding. It is often fatal if untreated.
The DRC has experienced 10 Ebola outbreaks since the virus was discovered on the Ebola River in 1976, and it has altogether killed some 900 people.
The United Nations children's agency UNICEF said an unusually high proportion of the people affected in this outbreak were children.
Two children have already died from the disease, and centres in Beni and Mangina were treating six othersinfected by the disease or suspected to be.
In a bid to halt the virus' advance, DRC health authorities said that doctors in Beni had begun using a prototype drug called mAb114 to treat patients.
Developed in the United States, the prototype drug is a protein that binds on to a specific target of the virus and triggers the body's immune system.
Ghebreyesus said five patients had received the unlicensed drug so far, and that WHO would like the roll-out "to speed up as much as possible".
Targeted vaccination, aimed primarily at front-line health workers, began last week, and so far 216 people have received a jab.
An epidemic between 2013 and 2016 killed more than 11,300 people in West Africa.
