The World Health Organization (WHO) said Friday it could not be certain it had identified all people 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.

More than 500 people have been vaccinated so far against the disease in Congo's latest outbreak, marked by a total of 78 confirmed and probable cases including 44 deaths, it said. Some 1,500 people have been identified as contacts of infected people.

"We don't know if we are having all transmission chains identified," WHO spokesman Tarik Jasarevic told a Geneva news briefing. "We expect to see more cases as a result of earlier infections and infection developing into illness."