The steep drop in the number of Ebola cases has suddenly flattened out, causing concern particularly about the situation in Guinea and Sierra Leone, according to the World Health Organization's (WHO) Ebola response team.

The current Phase II of the Ebola response to move from a slowing down of the outbreak to zero new cases will require new financing of $1.5bn. So far only a sum of $600m has been raised.

"One of the biggest risks we face right now is that new financing for this response is falling faster than the new cases. That's the reality," said Bruce Aylward, director-general for Ebola Response.

"The first three to four weeks of 2015 ... was incredibly encouraging because of the rapid decline in cases that we saw," Aylword said.

"We saw four weeks of week-on-week decline in cases. The story of the last four weeks has been different. Four weeks of relatively flat curve, frankly, where we're seeing about 120 cases to 150 cases a week. This is not what you want to see with Ebola."

Case finding and contact tracing have to become more effective to eliminate the virus.

"There is absolutely no question at this point that we have the strategy to get Ebola finished," the director said. "There's also absolutely no question that the infrastructure is in place as a result of the tremendous success of the international and national response."

"But, we do not have the financing necessary to get from there, get to zero, and do that safely and to get the job finished," he said.

Guinea and Sierra Leone are two countries "where we are going to have our toughest challenges", he said.

"We are deep into what we now refer to as Phase II of the Ebola Response," said David Nabarro, the UN secretary- general's special Ebola envoy. "Phase I was when the disease was way ahead of the response capacity, (and) escalating."

"The goal in phase I was to build infrastructure -- beds and burial teams -- to rapidly slow down the virus," he said. "Phase II was all about trying to move from that to zero" new cases."

Liberia to lift curfew

Following a drop in new cases to a tenth of the numbers being reported at the September-October peak of the outbreak, Liberia is lifting nationwide curfews and re-opening borders shut last year.

President Ellen Johnson Sirleaf said "health protocols" would prevent the importation of the virus through any of the re-opened crossing points to Guinea and Sierra Leone.

Almost 9,500 people have died in the outbreak.

The leaders of Liberia, Guinea and Sierra Leone have vowed to eradicate the virus by mid-April. It is hoped to stem the outbreak completely before the rains set in and hamper response efforts to reach remote parts.

The response in Sierra Leone is focused on its west including the capital Freetown, where the outbreak has yet to be brought under control.

Around 700 homes in Freetown were brought under quarantine last week following the death of a fisherman who tested positive for Ebola.

Transmission remains "widespread" in Sierra Leone, according to the WHO, which reported 74 new confirmed cases in the week to February 15, down from 76 the week before.