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Thirty-one new cases of Ebola have been reported in a growing geographic area in Guinea and Sierra Leone, according to the World Health Organization said.

At the beginning of this week, 14 additional cases were reported.

The latest figures mark the second straight week that the number of Ebola cases in West Africa has increased, officials said.

Investigators are working to trace how the latest cases of the disease were contracted, the WHO said.

"The outbreak is not over and the response efforts must be sustained until we get to zero cases throughout the region and are able to stay at zero for several months," the U.N. Mission for Ebola Emergency Response said Thursday.

Last month, officials from the WHO declared Liberia free of the disease. But even as they trumpeted the news, officials warned that outbreaks in Guinea and Sierra Leone ran the risk of bringing the virus back to Liberia, where more than 4,000 people died after contracting Ebola.