The World Health Organization is still waiting for its members to stump up another $4 million to tackle the growing threat from the Zika virus, WHO chief Margaret Chan said on Tuesday.

"The more we know, the worse things look," Chan told a news conference at WHO headquarters in Geneva. "In less than a year the status of Zika has changed from a mild medical curiosity to a disease with severe public health implications."

Zika has been linked to thousands of suspected cases of microcephaly, a rare birth defect, in Brazil, and a neurological disorder called Guillain-Barre Syndrome.

A pattern had emerged with initial virus circulation followed about three weeks later by an unusual increase in Guillain-Barre cases, then fetal malformations as pregnancies of infected women come to term, Chan said.

Zika has not been proven to cause Guillain-Barre or microcephaly, a condition characterized by unusually small heads in babies and linked with developmental problems. However, there is growing evidence that suggests a link to both disorders.

The WHO and its American arm PAHO have asked for $25 million to fight Zika and have received $3 million, and are now in an "active discussion" over the next $4 million, said Chan, who called the funding situation "pretty serious".

More on this...

She said she would shift money across the WHO budget as far as possible, but 80 percent of WHO's money was earmarked for specific causes.

WHO's strategy director Chris Dye said many, many millions of people had been exposed to the mosquito-borne virus, which has spread through most of the Americas in the past six months.

The latest medical studies suggested that perhaps 1 percent of infections would lead to severe neurological disorders, he said.

"If we just take that as an approximation, we know already that there are thousands of cases in just one part of Brazil, so the expectation across the Americas as a whole is many more thousands of cases."

Brazil has confirmed more than 860 cases of microcephaly, and considers most of them to be related to Zika infections in the mothers. It is investigating more than 4,200 additional suspected cases of microcephaly.