Herve Ludovic de Lys, head of the UN humanitarian coordination office in the region, says the outbreak needs an immediate response.
He warns the disease is likely to spread to Nigeria, Cameroon and Chad, as heavy rains pour over the region.
The worst of the epidemic hit Guinea-Bissau, which has seen 9,047 infections and 172 deaths between June and August, health authorities say.
De Lys says there has been widespread flooding in that area, forcing cholera cases to be transported to Bissau City, often times delaying treatment until it is too late.
In Liberia, nearly 4,000 cases are reported in Sinoe County, most in makeshift camps where illegal miners dig for diamonds. Typically, these camps lack clean drinking water and proper sanitation, creating the ideal conditions for the spread of cholera.
Death and infection are reported in Mauritania, Guinea, Senegal, Burkina Faso, Mali, Ivory Coast and Niger.
Cholera is very often passed through infected water, causing death by dehydration from diarrhea and vomiting.
It is treatable if patients are quickly rehydrated and clean water supplies are made available.
