The death toll from West Africa's Ebola outbreak has risen to more than 2,400 from at least 4,784 cases, but that is highly likely to be an underestimate, the World Health Organisation's director general Margaret Chan said on Friday.

Chan said the number of cases of the deadly viral disease is rising faster than authorities' ability to manage them, and she called for international support in sending healthcare workers, medical supplies and aid to the worst-affected countries of Guinea, Sierra Leone and Liberia.