Three babies have been born in the United States with birth defects linked to likely Zika virus infections, along with three cases of lost pregnancies.
The six cases reported as of June 9 have been included in a new US Zika pregnancy registry created by the Centers for Disease Control and Prevention.
The agency will begin regular reporting of poor outcomes of pregnancies with laboratory evidence of possible Zika virus infection in the 50 states and the District of Columbia.
Zika has caused alarm throughout the Americas since numerous cases of the birth defect microcephaly linked to the mosquito-borne virus were reported in Brazil, the country hardest hit by the current outbreak.
The rare birth defect is marked by unusually small head size and potentially severe developmental problems.
The US cases so far involve women who contracted the virus outside the United States in areas with active Zika outbreaks or were infected through unprotected sex.
There have not been cases reported of local transmission of the virus in the US.
Health experts expect local transmission to occur as mosquito season gets underway with warmer weather, especially in Gulf Coast states, such as Florida and Texas.
The CDC declined to provide details of the three cases it reported on Thursday but said all had brain abnormalities consistent with congenital Zika virus infection.
Two US cases of babies with microcephaly previously were reported in Hawaii and New Jersey.
The poor birth outcomes reported include those known to be caused by Zika, such as microcephaly and other severe foetal defects, including calcium deposits in the brain indicating possible brain damage, excess fluid in the brain cavities and surrounding the brain, absent or poorly formed brain structures and abnormal eye development, the CDC said.
"The pattern we're seeing here in the US among travellers is very similar to what we're seeing in other places like Colombia and Brazil," Dr Denise Jamieson, co-leader of the CDC Zika pregnancy task force, said.
Authorities in Brazil have confirmed more than 1400 cases of microcephaly in babies whose mothers were exposed to Zika during pregnancy.
