Speaking at a joint news conference in Geneva that was posted on the health organization’s website, they said some of the new cases in Guinea and Sierra Leone were patients who had not been on lists of people monitored for possible infection. These contact lists, used by epidemiologists, are a crucial part of the strategy to eradicate the disease.

“Good progress is being made, but the outbreak still presents a grave threat,” Dr. Nabarro said. “We have to really work hard to get to what we call zero-zero — zero cases, zero transmissions.”

Dr. Aylward said some of the new cases had emerged in people who had traveled far from their original points of infection, presenting additional complications on how to track all the people they might have infected — and who now may be Ebola transmitters themselves.

“Not only are we seeing slight increases in the virus but also seeing these long-distance movements, which are a challenge,” Dr. Aylward said.