Long-anticipated clinical trials of a possible Ebola vaccine will start soon in West Africa, according to a top US health official.

The news comes as the response to the outbreak took on added urgency with new cases in Mali and reports that the death toll has surpassed 5,000.

Results of an initial US safety study proved promising enough that next-step testing should begin in Liberia and Sierra Leone by January, Dr Anthony Fauci of the National Institutes of Health told a Senate committee on Wednesday.

If those new studies go well, "we could know by the middle of 2015 whether or not we have an effective vaccine," he said.

The Senate Appropriations Committee has begun evaluating the Obama administration's request for $6.2 billion in emergency aid to fight Ebola.

While the number of infections is slowing in some parts of West Africa, the World Health Organisation said cases are still surging in Sierra Leone, and in Mali, three deaths linked to Ebola were reported on Wednesday.

"That cluster has to be controlled or we're going to have another front," warned Dr Tom Frieden, director of the Centres for Disease Control and Prevention.

The spending request includes $4.64 billion in immediate money to fight the epidemic in West Africa while at the same time shoring up US preparedness.

The domestic work includes continuing training so far given to 250,000 nurses and other US health workers on how to safely handle any future patients, designating hospitals capable of handling Ebola or other serious infectious diseases and measures to create a national stockpile of protective equipment.