The epidemic is proving "exceptionally difficult to control and has spread across the whole country," said Veronique Taveau, spokesman for the UN children's fund (UNICEF).
Cholera is endemic in the country and flares up every year, but is currently "out of control, and risks spreading over into neighbouring countries," added Elisabeth Byrs, spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs.
There are 1,000 new cases of cholera every month, and the capital Bissau is the worst affected with over 7,600 cases, she added.
UNICEF said the epidemic is driven by three main factors: a dilapidated health system, a lack of access to clean drinking water, and a local funeral rite in which families drink the water that has been used to clean the deceased's body.
Cholera is an intestinal bacteria that causes serious diarrhoea and vomiting leading to dehydration. With a short incubation period, it can be fatal if not treated in time.