UN experts said  that the spread of the disease in Guinea, Liberia and Sierra Leone has dropped to almost 10 percent of what it was in September.
But UN Special Envoy on Ebola David Nabarro said the fight has reached a "second phase, getting to zero, and it really is the hardest part."
Ebola has infected nearly 24,000 people and killed more than 9,600, according to World Health Organization statistics. Nabarro said eradicating the disease requires treating all those who are ill and tracking everyone they come in contact with.
"It's a really difficult, painstaking task," he said in Brussels, on the eve of a major international conference on Ebola expected to draw around 600 delegates, including the leaders of Guinea, Liberia and Sierra Leone.
According to UN figures, the number of new cases has dropped from some 800-900 per week at the height of the outbreak to around 100 per week.
But the head of the UN agency charged with handling the emergency response, Ismail Ould Cheikh Ahmed, warned that plenty still needs to be done.
"We are reaching the most difficult phase, where there is a fatigue among the governments among the population, certainly among the donors, and we are not yet there," he said.
The outbreak is focused in an area of West Africa about the size of France, with a population of about 20 million people, and where infrastructure is limited.
Ebola has cost the three countries an estimated 12 percent of their gross domestic product, shifted resources causing other health care problems, and hurt business as people became scared to leave their homes or go to markets.

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