"The cholera outbreak continues with 3,915 cases and 90 deaths reported as of 31 August" in Guinea Bissua, said a World Health Organization report seen by AFP.
"New cases have been detected in Bijagos archipelago bringing the number of regions affected to 10 out of 11."
Aid groups MSF-Spain and OXFAM-US have mobilised teams to support a response, the report added.
In Guinea, the outbreak of the water-borne disease "is stabilising in Boke and Boffa prefectures.
"No cases were reported between 18 and 24 August from the 18 (out of 38) facilities that sent in their surveillance reports," the WHO report added.
The United Nations gave 400,000 euros (570,000 dollars) in aid to tiny Guinea-Bissau last month to fight the epidemic.
The money was partly to be used to help train health agents and for a public awareness campaign via the media.
Cholera causes serious diarrhoea and vomiting and can be fatal if not treated within 24 hours.