The steep decline in Ebola case numbers has levelled off over the past month and the development is a cause for concern, the official leading the World Health Organization's response to the outbreak said Friday.

Dr. Bruce Aylward told reporters "today is the first time we have the data to demonstrate this" flattening of the curve.

The United Nations has said 10 times fewer people are being diagnosed with Ebola each week than in September. Over the past four weeks, however, the line of the graph has flattened out, with the rate around 120 to 150 new cases a week.

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"It's what keeps me up at night right now," Aylward said. "This is not what you want to see with Ebola."

Health officials have expressed optimism in recent weeks that the tide seems to be turning in the fight against the deadliest Ebola outbreak in history. The presidents of the three worst affected countries, Liberia, Sierra Leone and Guinea, this week said they hope to reduce the number of new cases to zero by April 15.

But Aylward said that goal will be difficult to achieve.

The steep decline in case numbers came about as health workers worked hard for months to ensure safe burials for the dead and treatment beds for the sick. Now, for the final push to zero, the focus must swing to finding cases and tracing the personal contacts of each person infected with Ebola.

Aylward said these efforts are complicated by the "extraordinary measures" taken by people who do not want to be included on the contact list because of the stigma associated with the disease.

More than 23,300 have been infected by Ebola, and almost 9,500 people have died.

Aylward was meeting Friday with the United Nations Ebola chief, Dr. David Nabarro, on how to end the outbreak that began in December, 2013. They said they have the strategy to get it done, but financing remains a concern.

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Nabarro told reporters that about $1.5-billion is needed for 2015 but around $600-million of that has been raised so far.