The mosquito-born Zika virus may be responsible for an excess number of birth defects in U.S. states and territories where the virus had been circulating in local mosquito populations, even in women who had no lab evidence of Zika exposure during pregnancy, U.S. health officials said on Thursday.

Areas with local Zika transmission, including southern Florida, a portion of south Texas and Puerto Rico, saw a 21 percent increase in birth defects strongly linked with Zika during the last half of 2016 - when Zika was present - compared with the first half of that year, the U.S. Centers for Disease Control and Prevention reported.

Researchers said it is not clear whether the increase is due to local transmission of Zika alone, or other contributing factors.

The Zika outbreak was first detected in Brazil in 2015 and quickly spread through the Americas. It has been linked to thousands of suspected cases of microcephaly, a typically rare birth defect marked by unusually small head size, as well as eye abnormalities and nerve damage resulting in joint problems and deafness.

For the study, the CDC examined data from existing birth defect surveillance systems in 15 U.S. jurisdictions to look for birth defects possibly associated with Zika.

They broke these states and territories down into three groups - places with local Zika transmission, places with higher levels of travel-associated Zika, and places with lower rates of travel related Zika.

Overall, they found that an excess of three cases of birth defects per 1,000 live births that may have been linked with Zika virus infection out of a group of 1 million births in 2016.

Because many pregnant women exposed to Zika virus in late 2016 gave birth in 2017, CDC researchers anticipate that there could be another increase in possible Zika-related birth defects when 2017 data are analyzed.

“This report highlights the critical importance of documenting birth defects possibly related to Zika and our need to maintain vigilance,” CDC Director Dr. Brenda Fitzgerald said in a statement.