A decline in Ebola cases in West Africa must not result in a loss of vigilance because a single new case is enough to reignite an outbreak, international medical charity Medecins Sans Frontieres (MSF) said on Monday.

The downward trend of new cases in Liberia, Guinea and Sierra Leone should be used as an opportunity to focus efforts on addressing weaknesses in the response, otherwise progress made in stemming the deadly virus would be jeopardized, it said.

"We are on the right track, but reaching zero cases will be difficult unless significant improvements are made in alerting new cases and tracing those who have been in contact with them," Brice de la Vingne, MSF head of operations, said in a statement.

"A single new case is enough to reignite an outbreak."

In the past week fewer than 150 cases of disease were reported, according to the World Health Organization (WHO).

Sierra Leone remains hardest-hit, accounting for 117 of the 145 new confirmed cases, against 184 there the previous week and 248 the week before that, the WHO said on Thursday.

MSF said that there was almost no information sharing between the three worst-affected countries about tracing people who might have been in contact with Ebola patients.

"With people moving frequently across borders, it is essential that the surveillance teams based in each country collaborate immediately so that new cases are not imported into areas considered Ebola-free," said de la Vingne.

MSF said that there were now just over 50 patients in its Ebola management centers across Guinea, Liberia and Sierra Leone.

It said although cases were decreasing, hot spots persisted, for example in Sierra Leone's capital Freetown.

On Sunday WHO said it would create a contingency fund and an emergency workforce to respond quickly to crises after criticism of the agency's response to the Ebola epidemic.