Three of 12 samples sent to a reference laboratory in Senegal showed Zika but could not determine any link to the virus' recent outbreak in the Americas and the western Pacific, World Health Organisation spokesman Christian Lindmeier said.
The agency has been in contact with Guinea Bissau's government, and has previously warned that any country where the Zika-carrying Aedes aegypti mosquito is prevalent could be at risk for Zika spread.
Sixty-one countries and territories had reported continuing mosquito-borne transmission of the virus, WHO said in its latest situation report released Thursday on Zika. The Geneva-based agency has called the latest outbreak a "public health emergency of international concern.