Dozens of new Ebola cases reported in West Africa

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At least 45 new Ebola cases have been reported in West Africa this month

"The outbreak is not over and the response efforts must be sustained," a U.N. mission says

CNN  — 

It seemed like the number of people contracting Ebola in West Africa was on the decline. But now, officials say that trend has stalled, with dozens of new cases of the deadly virus reported so far this month.

Last week, there were 31 new cases of Ebola reported in a growing geographic area in Guinea and Sierra Leone, 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.

CNN’s Debra Goldschmidt and Azadeh Ansari contributed to this report.