Evidence is growing of a link between the Zika virus and potential birth defects in babies, the World Health Organization said on Friday, though this could take four to six months to prove.

In autopsies, doctors in Brazil have found evidence of the virus in the brains of babies born with the neurological disorder microcephaly, said the director of the WHO's outbreaks and health emergencies department, Dr. Bruce Aylward.

"This does not still prove causation... but it is an increasing accumulation of evidence in terms of the temporal-geographic association of the virus and the consequences that we are worried about," he told a news briefing. "We are seeing a lack of other explanatory causes."

The WHO declared the Zika outbreak, which has spread to nearly 30 countries and territories, an international health emergency on Feb. 1.

Aylward said it would move early next month to accelerate research on developing diagnostic tests and vaccines against Zika by convening international experts. This was "very similar to what we did early days of Ebola," he said.

Microcephaly is a neurological condition marked by unusually small heads that can result in developmental problems.

Brazil said it has confirmed more than 500 cases, and considers most to be related to Zika infections in the mothers. It is investigating more than 3,900 additional suspected cases.

Pregnant women in Colombia confirmed as being infected by the Zika virus will deliver babies in coming months, which is an opportunity for gathering further evidence, Aylward said.

"It will probably be somewhere (around) 4 to 5 or 6 months before we can say with some certainty," he said of the link.