The U.N. health agency says it and national authorities are investigating whether three cases of the Zika virus discovered in Guinea Bissau are of the same strain as the one behind outbreaks linked to head and brain abnormalities in Brazil and elsewhere.

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 Organization spokesman Christian Lindmeier said.

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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. WHO has reported the latest Zika strain was found in Cape Verde, a group of islands off Africa's Atlantic coast and a former Portuguese colony like Guinea Bissau and Brazil.

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."