GENEVA — The World Health Organization on Thursday declared an end to the Ebola virus outbreak in West Africa after no new cases emerged in Liberia, while cautioning that the world cannot yet be fully declared free of the disease that claimed more than 11,300 lives during a two-year outbreak.

Liberia, which along with Sierra Leone and Guinea was an epicenter of the latest outbreak, was first declared free of the disease last May, but new cases emerged two times — forcing officials there to restart the clock.

"While this is an important milestone and a very important step forward, we have to say that the job is still not done," said Rick Brennan, WHO director of emergency risk assessment and humanitarian response, at a news conference in Geneva. "That's because there is still ongoing risk of re-emergence of the disease because of persistence of the virus in a proportion of survivors."

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Dr. Bruce Aylward, WHO's special representative for the Ebola Response, said in a statement: "The risk of re-introduction of infection is diminishing as the virus gradually clears from the survivor population, but we still anticipate more flare-ups and must be prepared for them."

Brennan said the virus can persist in the body fluids of some survivors — and importantly, in the semen of male survivors for a period of up to 12 months. He said WHO estimates that there are about 1,200 survivors in Guinea, 5,000 in Sierra Leone and 4,000 in Liberia.

Related: Ebola Returns to Liberia, Again

The virulent disease first emerged in rural Guinea in December 2013, then spread to the country's capital and across borders to Liberia and Sierra Leone. Cases also popped up in Mali, Senegal and Nigeria though transmission chains there were quickly shut down.

A Liberian man who traveled to the United States in October 2014 fell ill and died in Texas of Ebola. No other cases emerged there, though his death sparked widespread panic in the United States.