Zika virus has been confirmed as a cause of the birth defect microcephaly and other serious birth defects, according to the U.S. Centers for Disease Control and Prevention.

The CDC's findings are published in a special report in Wednesday's New England Journal of Medicine.

"This is a study that marks a turning point in the Zika outbreak. It is now clear, the CDC has concluded, that Zika does cause microcephaly," CDC director Dr. Tom Frieden said in teleconference.

Babies with microcephaly are born with abnormally small heads that can result in developmental problems.

The conclusion isn't based on a single piece of conclusive proof of a connection. Rather, the call was based on mounting evidence from several studies and a careful review of the criteria needed to determine if something causes birth defects, the agency said.

"There is still a lot that we don't know but there is no longer any doubt that Zika causes microcephaly. We're undertaking further study to look at the spectrum of disorder that the virus may cause."

'Messages will now be more direct'

The U.S. public health authority is not changing its precautionary advice to travellers, such as to prevent mosquito bites and sexual transmission. Those public health messages were based on preventing mosquito bites in general since the insects also transmit other viruses such as dengue and yellow fever.

"I think our messages will now be more direct," Dr. Sonja Rasmussen, CDC's director of the division of public health information and dissemination said. She hopes the news will prompt people to take precautions, given surveys suggest many Americans aren't concerned or know much about it.

An edes aegypti mosquito is seen inside a test tube as part of a research on preventing the spread of the Zika virus and other mosquito-borne diseases at a control and prevention centre in Guadalupe, neighbouring Monterrey, Mexico, in this March 8, 2016 file photo. (Daniel Becerril/Reuters)

Not all babies will have problems, said Rasmussen, who led the latest research. Studies are underway to try to determine which proportion could be.

The researchers based their declaration on two approaches. The first was finding cases with a combination of a documented exposure to Zika virus in pregnancy and birth of babies with microcephaly in multiple studies. This was combined with finding patterns of complications in different regions, said Dr. Isaac Bogoch, an infectious disease specialist at Toronto General Hospital.

The mounting evidence also included clinical findings of the virus in pregnant women and microscopic findings from fetal tissue.

Microcephaly and other fetal malformations that might be associated with Zika infection have been reported in more than 1,000 cases in Brazil, seven in Colombia, eight previously in French Polynesia as well as smaller numbers in Martinique, Panama and Cabo Verde, the World Health Organization said last week.

Since 2007, mosquito-borne Zika virus cases have been reported in 17 countries and areas of the Western Pacific, the United Nations public health agency said.

Therapist Rozely Fontoura holds Juan Pedro, who has microcephaly, in Recife, Brazil March 26, 2016. (Paulo Whitaker/Reuters)

For the U.S., the report comes as health officials submit an emergency request for $1.9 billion US to fight Zika internationally and prepare in case mosquitoes spread the virus within the country.

While parts of the continental U.S. may have the right climate and mosquitoes for Zika to spread, Bogoch cautioned that's not enough.

"It takes lot more than suitability for an infection to spread," Bogoch said.

Factors such as living conditions, presence of freestanding water around homes for mosquitoes to breed and lack of air conditioning also matter, he said.

There are no documented cases of Zika infections in Canada or the continental U.S. from local mosquitoes. Local mosquito transmission has been reported in Puerto Rico, the U.S. Virgin Islands, American Samoa. Widespread outbreaks are reported in Brazil and Colombia.