US President Barack Obama, flanked by military and civilian health workers, delivers remarks about the progress made to date in the response to the Ebola outbreak in West Africa, from the Eisenhower Executive Office Building on the White House campus in Washington, Feb 11, 2015. [Photo/Agencies]

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 civilians still fighting the disease in West Africa who are supported by the United States are essential to containing it by helping to trace Ebola victims' contacts, re-establish health services, change behavior in communities and study the disease.

"This is what's needed now as we move from the current situation toward zero transmission, which is our ultimate goal," he said. "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 the US 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.'"

Officials said the US 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.