The Ebola outbreak in the Democratic Republic of Congo has spread southward in an area of high security risks, the World Health Organization (WHO) said.
The outbreak, the worst in the country, has killed 439 of the 713 people believed to have contracted the disease. The fight against Ebola depends on tracking people who may have had contact with the disease and can get sick and spread further.
But the outbreak in a region of Congo with frequent fighting makes it difficult for health professionals to move and monitor potential patients and to spread messages about avoiding illness.
Most of the cases since the beginning of the year were in the Katwa Health Zone, where WHO said that Ebola workers faced "community mistrust" and that most people fell ill were not on the lists of people suspected of contacting or Ebola.
"The outbreak also extended south to Kayina's health zone, a high-risk area for safety," WHO said in its statement on Thursday. There were five cases in Kayina, which lies between the main outbreak area and the large city of Goma, which is near the border with Rwanda.
The WHO said that after conducting an Ebola simulation exercise in Rwanda, it was sending a team to strengthen the country's preparedness and vaccinate health workers who would be the first to contact Ebola if it spread across the border.
However, WHO spokeswoman Fadela Chaib said there has also been a decline in cases around the previous site, Beni.
"It's too early to shout victory, it's true we had some success in Beni because every step we took had an impact, but unfortunately we see cases popping up in other areas," she said.
"The country is not only facing Ebola, but other health threats, just to mention malaria, cholera, vaccine-derived polio and a long humanitarian crisis and violence in many regions."
More than 60,000 people have been vaccinated in the Congo, as well as 2,500 in Uganda, one of the countries with "very high" risk of the disease.
Chaib said there were 4,000 people with potential contact with ebola under surveillance and 156 patients in the hospital.