UNITED NATIONS -- World leaders Friday pledged $3.4 billion to fight Ebola, a viral disease that ravaged the populations of three African countries last year, then subsided after an unprecedented international effort -- but resurfaced in recent weeks with alarming vigor.

"The strategy to end the outbreak is working -- but the final stretch of the response remains particularly challenging," said UN Secretary-General Ban Ki-moon, host of the International Ebola Recovery Conference. "Cases in Guinea and Sierra Leone have been reduced considerably. The response is being fine-tuned to focus on increasing engagement, awareness and contact-tracing in the remaining affected communities."

Helen Clark, administrator of the UN Development Program, announced that donors had pledged more than the $3.2 billion sought to help buttress devastated health delivery and social services institutions in Guinea, Sierra Leone and Liberia, the three countries hardest hit by Ebola.

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"We feel this puts the recovery off to a very positive start," Clark said after tallying the pledges.

Less than two months after being declared "Ebola-free," Liberia reported new cases of Ebola on June 29, and Sierra Leone and Guinea also have seen new cases, experts said.

The pledges made Friday bring to more than $5 billion the funds devoted to assisting the affected countries in their multipronged fight against Ebola. The disease sparked panic even in the United States last year when U.S.-based health care workers contracted it while assisting the three affected countries. A Liberian man who had traveled to the United States died of the disease in a Texas hospital in October.

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The high-level summit brought to Manhattan the presidents of each of the affected countries and the president of the African Union, the European Union, World Bank, African Development Bank and Islamic Development Bank, among other dignitaries and financiers.

It comes on the heels of two similar conferences in Brussels in March and Washington in April and as Ebola cases -- which UN officials said killed as many as 11,000 people, mostly in West Africa -- now appear at a rate of about 30 each week, according to David Nabarro, Ban's special envoy on Ebola."We are not out of the woods yet because Ebola cases return even when we are down to zero," said Christos Stylianides, the European Union's Ebola coordinator and European commissioner for humanitarian aid and crisis management. "New cases in Liberia are a stark reminder of this."

In December, the UN Security Council passed a resolution declaring Ebola a "threat to international peace and security," which galvanized international efforts to stamp it out.

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"The impacts of the Ebola crisis have been far-reaching and much work is needed to support the countries," Ban said. "The outbreak has eroded progress on peace and development. It has disrupted health and social services," and economic sectors including agriculture, mining, trade, tourism, transport, fisheries and livestock.

"The African Development Bank will continue to work with the three affected countries," said Janvier Litse, acting vice president for Country and Regional Programmes and Policies. He added that the goal is "getting to zero and sustaining the zero."

Ellen John Sirleaf, Liberia's president, who spoke on behalf of heads of state in the Mano River region, thanked the international community for the support, but urged that still more was needed.

"There is no doubt that the resources required are significant," she said.