The United Nations is now planning for its Ebola response to last into mid-2016, suggesting the battle against the virus that has killed more than 11,000 people won't be over by year-end, according to a report Wednesday.

The news comes as health officials struggle to contain a resurgence of cases in both Sierra Leone and Guinea largely because of problems tracing patients.

The World Health Organization and its humanitarian partners indicated Wednesday that Phase Three of their Ebola response would continue into 2016. Those activities include stopping transmission of Ebola, indicating more cases are anticipated.

Liberia, which had the most Ebola deaths, has managed to stop the virus. By contrast, Guinea continues to report new cases nearly two years after the virus first emerged in the country's remote forests.

As many as 11 percent of contacts are still not being traced on any given day, Wednesday's report said.

"One of the new reported cases was symptomatic within the community for two weeks followed by trips to two clinics in the capital before detection," it said. "Efforts are underway to strengthen contact tracing, which continues to be an issue."

Sierra Leone is also struggling to track the origin of its cases, giving the sick more time to interact with and potentially infect others. Earlier this month five new patients emerged including a 16-year-old girl who was the first case in her area in more than five months. It's still not known how she got sick, health officials said.

Dr. Margaret Chan, the head of the World Health Organization, had told the U.N. Security Council back in August that the Ebola epidemic could be over by the end of 2015. At the same time, though, she also cautioned against "a false sense of security" and said setbacks were possible.