The World Health Organization (WHO) says it has identified 1,500 people who have potentially come into contact with Ebola patients in the east of the Democratic Republic of Congo, but says its staff cannot reach some areas because of the conflict.
During an Ebola outbreak it is critical for health workers to find all the people who may have come into contact with the virus - they are then carefully monitored each day in case they show symptoms including a high fever.
The WHO says there are potentially more cases in those inaccessible areas.
A WHO spokesman said 44 people have died during the current outbreak in North Kivu Province, a volatile area where various rebel groups are active.
Earlier this week the charity Médecins Sans Frontières (MSF) opened an Ebola treatment centre in Mangina, the small town which is the epicentre of the outbreak.
MSF says ongoing violence and displacement of civilians in the area are having a significant impact on the humanitarian response.