The World Health Organization says it suspects a link between the mosquito-borne Zika virus and a rare birth defect that gives babies abnormally small heads but says so far the evidence is circumstantial.

WHO spokesman Christian Lindmeier says the "big task" is to try to establish a link between the virus and microcephaly, which involves abnormally small heads in newborns and can affect brain development.

He said the U.N. agency plans a special session Thursday on the virus during a Geneva meeting of its executive board. He said the virus has been associated with close to 4,000 microcephaly cases in Brazil, and El Salvador, Panama, Colombia and Cape Verde also have "large outbreaks."

Lindmeier told reporters Tuesday the "huge increase" of Zika cases "gives a lot of reason for concern."