West Africa can expect flare-ups of Ebola in the coming year even if the world's worst outbreak of the disease will be declared effectively over, UN chief Ban Ki-moon said Wednesday.

The World Health Organization on Thursday will declare Liberia Ebola-free, joining Guinea and Sierra Leone, which earned that status at the end of last year.

For the first time since the outbreak began in December 2013, all three countries will have registered zero cases in at least 42 days, which is twice the incubation period for the virus.

But in an address to the General Assembly, Ban warned that the virus could resurface, if in limited fashion.

"We can anticipate future flare-ups of Ebola in the coming year," Ban said. "But we also expect the potential and frequency of those flare-ups to decrease over time."

World Health Organization director Margaret Chan said the virus can persist in some Ebola survivors even after they have fully recovered.

Since March of last year, there have been 10 minor flare-ups of infections.

"By the end of this year, we expect that all survivors -- all survivors -- will have cleared the virus from their bodies," Chan said.

More than 28,600 people have been infected by the virus in West Africa and 11,300 have died.

Chan described the next three months as "the most critical," as foreign medical groups shut down operations in West Africa and national health ministries take over, managing disease surveillance and response.

The Ebola outbreak overwhelmed the weak health care systems in the three countries and delivered a major blow to their economies, prompting calls for aid.

Recovery will take time, Chan told the 193-nation assembly, but she asserted that there would be no return to a full-blown epidemic.

"No one will let this virus take off and run away again," she said.