WASHINGTON (Reuters) - The U.S. Centers for Disease Control and Prevention said on Thursday that it is monitoring 320 U.S. pregnant women with laboratory evidence of Zika virus infection, up from 287 women a week earlier.

However, the number of babies born in the United States with birth defects linked to Zika infection in mothers during pregnancy, or lost pregnancies linked to the virus, remained unchanged from last week's report at 7 and 5, respectively, according to a CDC registry created last month.

The registry compiles poor outcomes of pregnancies with laboratory evidence of possible Zika virus infection in the 50 states and the District of Columbia. The latest figures are as of June 30.

Zika has caused concern throughout the Americas due to an alarming rise in cases of the birth defect microcephaly and other severe fetal brain abnormalities linked to the mosquito-borne virus reported in Brazil, the country hardest hit by the outbreak. Infants with microcephaly are born with abnormally small heads and may experience potentially disabling developmental problems.

Brazil has confirmed more than 1,600 cases of microcephaly linked to Zika.

All reported U.S. cases of Zika have so far involved people who traveled to areas with a current outbreak, but health experts have warned that local transmission cases are likely to occur in the coming weeks during summer mosquito season. Gulf Coast states, such as Florida and Texas, are seen as particularly vulnerable.

The virus can also be transmitted via unprotected sex with an infected man.

(Reporting by David Morgan in Washington and Bill Berkrot in New York)