In February, the World Health Organization declared Zika a global health emergency based on its suspected link to thousands of cases in Brazil of microcephaly, a birth defect marked by small head size and underdeveloped brains.

The declaration kicked off a flurry of studies to prove a link. The CDC said its latest conclusions came after all necessary scientific criteria had been met to make the official call.

"The data are there. The evidence is there. The pieces of information we have now makes us confident," said Dr. Sonja Rasmussen, director of the CDC division of public health information and lead author of a New England Journal of Medicine article outlining evidence.

CDC now believes microcephaly is just one of a range of serious birth defects caused by Zika. In Brazil, officials have confirmed more than 1,100 cases of microcephaly, and considers most of them to be related to Zika infections in the mothers. It is investigating more than 3,800 additional suspected cases.

CDC travel and sexual transmission guidelines remain unchanged. Women who are pregnant or planning to become pregnant are advised to avoid travel to the at least 42 countries and territories where Zika has spread, and men who have been to those areas are advised to abstain from sex or use condoms with partners who are pregnant or trying to become pregnant.

There were already signs on Wednesday that Republican lawmakers' resolve against funding the White House Zika request is weakening.

Senior US House of Representatives Republican Tom Cole said on Wednesday more funds will be needed to fight the Zika virus in the United States, signaling a shift from insistence by many Republicans that the Obama administration should use existing funds for the effort to combat the growing threat.

"There's going to need to be additional money, I don't think there's any doubt about that," Cole told reporters after a House Republican meeting. "We're having discussions about that now."

Republican Senator Marco Rubio, who is from Florida, broke with other congressional Republicans on Zika last week, announcing his support for President Barack Obama's nearly $1.9 billion request to fight the virus.

On Wednesday, Rubio wrote to the CDC to urge it to clear a backlog of Zika diagnostic tests and prioritize testing for pregnant women, saying he had seen media reports that some pregnant women have waited up to a month for CDC to complete their tests.

House Speaker Paul Ryan stood fast, however, saying that if more money is needed to fight Zika, lawmakers will respond through the regular appropriations process.

Now that the causal relationship has been established, Frieden said several important questions must be answered, such as what percentage of Zika-infected mothers have babies with birth defects. Researchers also want to discover the full range of brain and developmental issues that may crop up later in life for infected babies, Rasmussen said.