WASHINGTON (AP) -- President Barack Obama heralded "new phase in the fight" against Ebola on Wednesday and said progress against the outbreak in West Africa will allow the U.S. to withdraw nearly all American troops sent to Liberia last fall.

TUBMANBURG, LIBERIA - JANUARY 29: A health worker departs a high-risk section of the Ebola Treatment Unit (ETU), on January 29, 2015 near Tubmanburg, Liberia. (John Moore, Getty Images)

He cautioned the mission was not over, and he set an ambitious goal of eliminating the disease.

"We have risen to the challenge," he said at the White House. "Our focus now is getting to zero."

Obama said only 100 of the 2,800 troops sent to Liberia will remain there after April 30. About 1,500 have returned home. Those staying will work with Liberia's military, regional partners and U.S. civilians.

Obama's upbeat announcement, made with military responders and Ebola survivors at his side, was a significant turnabout from last year when the White House's initial response to the outbreak was criticized as inept and too slow.

Back then, Obama resisted calls to impose a travel ban and was forced to cancel midterm campaign appearances to stay in Washington and focus on Ebola, particularly after health workers contracted the virus at a Texas hospital while treating a man who was infected in Africa.

The U.S. tightened policies at home and dedicated more resources to West Africa.

"People were understandably afraid," Obama said. "Some stoked those fears."

Earlier Wednesday, he met with philanthropists and foundation leaders who had supported the fight against the outbreak, which had threatened to spiral out of control and fostered fears in the U.S. and elsewhere beyond West Africa.

The U.S. pullout comes as Ron Klain, who led Obama's Ebola response, wraps up his short-term assignment.

At the height of the outbreak, Liberia was experiencing 119 confirmed Ebola cases per week. This week there were only three.

But Guinea reported a sharp increase with 65 new confirmed cases compared with 39 the week before. Sierra Leone reported 76 new confirmed cases.

"What we're seeing in Guinea and in Sierra Leone is that the new cases are not cases that are showing up on known contacts lists," said J. Stephen Morrison, senior vice president and director of the Global Health Policy Center at the Center for Strategic and International Studies. "The transmission is coming from somewhere else and we don't know where that somewhere else is."

Pointing to the disappointing rise in cases, Dr. David Nabarro, the United Nations' Ebola chief, warned in an interview with The Associated Press that the battle against Ebola is far from over. He said the more than 10,000 American civilians still fighting the disease in West Africa are essential to containing it.

"I think the situation now is that these American forces that came out to do a particular job have done that job, and that's why the U.S. government has started to withdraw them," Nabarro said. "This kind of massive assistance, put in at that time, contributed to the rapid decline in numbers of people with Ebola."

He said the U.S. civilians currently working in the region are helping to trace Ebola victims' contacts, re-establish health services, change behavior in communities and study the disease "and this is what's needed now as we move from the current situation towards zero transmission, which is our ultimate goal."

"Without that, the sustained high level of backing right through to the very end of this outbreak, we could end up in the embarrassing situation of seeing rebound, which means that we see suddenly cases start to rise again because we've not managed to maintain the hard effort," he said.

Morrison, who worked at USAID and the State Department during the Clinton administration, said that without the boost from the U.S. and British militaries in the region, "we would have faced a complete runaway outbreak and a complete unraveling of society which was well on its way."

While careful not to declare the crisis over, Obama promoted the decline in Ebola cases as a sign that U.S. and global efforts had paid off.

Officials said the U.S. helped build 15 Ebola treatment units, trained more than 1,500 health workers and coaxed the world community into contributing more than $2 billion to Ebola efforts.

The outbreak has killed more than 9,100 people, and the World Health Organization has warned it will be challenging to cut the number of cases to zero. The outbreak is expected to cost the three most-affected countries -- Liberia, Sierra Leone and Guinea -- at least $1.6 billion in lost economic growth.

 

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