The Ebola outbreak in the Democratic Republic of the Congo (DRC) is expected to last another three to four months and could spread at any time to Uganda or Rwanda, the World Health Organization (WHO) said.
The WHO said on Thursday that the neighbouring countries are well prepared but have not yet approved the use of a vaccine.
The most concerning area is the city of Beni in the DRC's North Kivu province, where dozens of people who may have been exposed to the disease are hiding from health workers, WHO emergency response chief Peter Salama told Reuters news agency.
Salama also disclosed that the clinical trial of therapeutic Ebola treatment is under discussion and could begin in weeks.
The rate of new Ebola cases has more than doubled since September after rebel violence in northeastern DRC caused response efforts to be briefly suspended, health officials said.
The International Rescue Committee said it was "alarmed" that there were 33 new cases between Oct. 1 and Tuesday, versus 41 cases during all of September.
"This is a sign not only that the outbreak is not under control, but that without full engagement from the community things could get a lot worse," said Michelle Gayer, the IRC's senior director of emergency health.
Most of the new cases have been in Beni, where experts had to suspend Ebola containment efforts for days after a deadly rebel attack.
With multiple armed groups active in the region, health officials have said they are effectively operating in a war zone.
Earlier this week, the WHO noted that all of the health workers who have caught Ebola in this epidemic - 19 so far - have been infected outside of hospitals or clinics, meaning that the virus is spreading in the community.